diff --git a/cmd/pipecd/server.go b/cmd/pipecd/server.go
index d8bfb09014..2f30bbf400 100644
--- a/cmd/pipecd/server.go
+++ b/cmd/pipecd/server.go
@@ -199,7 +199,7 @@ func (s *server) run(ctx context.Context, input cli.Input) error {
 				datastore.NewPipedStore(ds),
 				input.Logger,
 			)
-			service = grpcapi.NewPipedAPI(ctx, ds, sls, alss, las, cmds, statCache, cmdOutputStore, cfg.Address, input.Logger)
+			service = grpcapi.NewPipedAPI(ctx, ds, sls, alss, las, cmds, statCache, rd, cmdOutputStore, cfg.Address, input.Logger)
 			opts    = []rpc.Option{
 				rpc.WithPort(s.pipedAPIPort),
 				rpc.WithGracePeriod(s.gracePeriod),
diff --git a/pkg/app/api/grpcapi/grpcapi.go b/pkg/app/api/grpcapi/grpcapi.go
index 9e06fc64e6..0b1eed7482 100644
--- a/pkg/app/api/grpcapi/grpcapi.go
+++ b/pkg/app/api/grpcapi/grpcapi.go
@@ -18,6 +18,7 @@ import (
 	"context"
 	"encoding/base64"
 	"errors"
+	"fmt"
 
 	"go.uber.org/zap"
 	"google.golang.org/grpc/codes"
@@ -189,3 +190,7 @@ func getEncriptionKey(se *model.Piped_SecretEncryption) ([]byte, error) {
 		return nil, status.Error(codes.FailedPrecondition, "The piped does not contain a valid encryption type")
 	}
 }
+
+func makeUnregisteredAppsCacheKey(projectID string) string {
+	return fmt.Sprintf("HASHKEY:UNREGISTERED_APPS:%s", projectID)
+}
diff --git a/pkg/app/api/grpcapi/piped_api.go b/pkg/app/api/grpcapi/piped_api.go
index 04b4400883..022ace071b 100644
--- a/pkg/app/api/grpcapi/piped_api.go
+++ b/pkg/app/api/grpcapi/piped_api.go
@@ -15,7 +15,9 @@
 package grpcapi
 
 import (
+	"bytes"
 	"context"
+	"encoding/gob"
 	"encoding/json"
 	"errors"
 	"time"
@@ -33,9 +35,11 @@ import (
 	"github.com/pipe-cd/pipe/pkg/app/api/stagelogstore"
 	"github.com/pipe-cd/pipe/pkg/cache"
 	"github.com/pipe-cd/pipe/pkg/cache/memorycache"
+	"github.com/pipe-cd/pipe/pkg/cache/rediscache"
 	"github.com/pipe-cd/pipe/pkg/datastore"
 	"github.com/pipe-cd/pipe/pkg/filestore"
 	"github.com/pipe-cd/pipe/pkg/model"
+	"github.com/pipe-cd/pipe/pkg/redis"
 	"github.com/pipe-cd/pipe/pkg/rpc/rpcauth"
 )
 
@@ -58,13 +62,14 @@ type PipedAPI struct {
 	deploymentPipedCache cache.Cache
 	envProjectCache      cache.Cache
 	pipedStatCache       cache.Cache
+	redis                redis.Redis
 
 	webBaseURL string
 	logger     *zap.Logger
 }
 
 // NewPipedAPI creates a new PipedAPI instance.
-func NewPipedAPI(ctx context.Context, ds datastore.DataStore, sls stagelogstore.Store, alss applicationlivestatestore.Store, las analysisresultstore.Store, cs commandstore.Store, hc cache.Cache, cop commandOutputPutter, webBaseURL string, logger *zap.Logger) *PipedAPI {
+func NewPipedAPI(ctx context.Context, ds datastore.DataStore, sls stagelogstore.Store, alss applicationlivestatestore.Store, las analysisresultstore.Store, cs commandstore.Store, hc cache.Cache, rd redis.Redis, cop commandOutputPutter, webBaseURL string, logger *zap.Logger) *PipedAPI {
 	a := &PipedAPI{
 		applicationStore:          datastore.NewApplicationStore(ds),
 		deploymentStore:           datastore.NewDeploymentStore(ds),
@@ -82,6 +87,7 @@ func NewPipedAPI(ctx context.Context, ds datastore.DataStore, sls stagelogstore.
 		deploymentPipedCache:      memorycache.NewTTLCache(ctx, 24*time.Hour, 3*time.Hour),
 		envProjectCache:           memorycache.NewTTLCache(ctx, 24*time.Hour, 3*time.Hour),
 		pipedStatCache:            hc,
+		redis:                     rd,
 		webBaseURL:                webBaseURL,
 		logger:                    logger.Named("piped-api"),
 	}
@@ -992,8 +998,25 @@ func (a *PipedAPI) UpdateApplicationConfigurations(ctx context.Context, req *pip
 }
 
 func (a *PipedAPI) ReportUnregisteredApplicationConfigurations(ctx context.Context, req *pipedservice.ReportUnregisteredApplicationConfigurationsRequest) (*pipedservice.ReportUnregisteredApplicationConfigurationsResponse, error) {
-	// TODO: Make the unused application configurations cache up-to-date
-	return nil, status.Errorf(codes.Unimplemented, "ReportUnregisteredApplicationConfigurations is not implemented yet")
+	projectID, pipedID, _, err := rpcauth.ExtractPipedToken(ctx)
+	if err != nil {
+		return nil, err
+	}
+
+	// Cache an encoded slice of *model.ApplicationInfo.
+	var buf bytes.Buffer
+	enc := gob.NewEncoder(&buf)
+	if err := enc.Encode(req.Applications); err != nil {
+		a.logger.Error("failed to encode the unregistered apps", zap.Error(err))
+		return nil, status.Error(codes.Internal, "failed to encode the unregistered apps")
+	}
+	key := makeUnregisteredAppsCacheKey(projectID)
+	c := rediscache.NewHashCache(a.redis, key)
+	if err := c.Put(pipedID, buf.Bytes()); err != nil {
+		return nil, status.Error(codes.Internal, "failed to put the unregistered apps to the cache")
+	}
+
+	return &pipedservice.ReportUnregisteredApplicationConfigurationsResponse{}, nil
 }
 
 // CreateDeploymentChain creates a new deployment chain object and all required commands to
diff --git a/pkg/app/api/grpcapi/web_api.go b/pkg/app/api/grpcapi/web_api.go
index 7a31a3b35b..a58ef1ac22 100644
--- a/pkg/app/api/grpcapi/web_api.go
+++ b/pkg/app/api/grpcapi/web_api.go
@@ -15,9 +15,12 @@
 package grpcapi
 
 import (
+	"bytes"
 	"context"
+	"encoding/gob"
 	"errors"
 	"fmt"
+	"sort"
 	"strings"
 	"time"
 
@@ -66,6 +69,7 @@ type WebAPI struct {
 	pipedProjectCache      cache.Cache
 	envProjectCache        cache.Cache
 	insightCache           cache.Cache
+	redis                  redis.Redis
 
 	projectsInConfig map[string]config.ControlPlaneProject
 	logger           *zap.Logger
@@ -102,6 +106,7 @@ func NewWebAPI(
 		pipedProjectCache:         memorycache.NewTTLCache(ctx, 24*time.Hour, 3*time.Hour),
 		envProjectCache:           memorycache.NewTTLCache(ctx, 24*time.Hour, 3*time.Hour),
 		insightCache:              rediscache.NewTTLCache(rd, 3*time.Hour),
+		redis:                     rd,
 		logger:                    logger.Named("web-api"),
 	}
 	return a
@@ -599,6 +604,53 @@ func (a *WebAPI) validatePipedBelongsToProject(ctx context.Context, pipedID, pro
 	return nil
 }
 
+func (a *WebAPI) ListUnregisteredApplications(ctx context.Context, _ *webservice.ListUnregisteredApplicationsRequest) (*webservice.ListUnregisteredApplicationsResponse, error) {
+	claims, err := rpcauth.ExtractClaims(ctx)
+	if err != nil {
+		a.logger.Error("failed to authenticate the current user", zap.Error(err))
+		return nil, err
+	}
+
+	// Collect all apps that belong to the project.
+	key := makeUnregisteredAppsCacheKey(claims.Role.ProjectId)
+	c := rediscache.NewHashCache(a.redis, key)
+	// pipedToApps assumes to be a map["piped-id"][]byte(slice of *model.ApplicationInfo encoded by encoding/gob)
+	pipedToApps, err := c.GetAll()
+	if errors.Is(err, cache.ErrNotFound) {
+		return &webservice.ListUnregisteredApplicationsResponse{}, nil
+	}
+	if err != nil {
+		a.logger.Error("failed to get unregistered apps", zap.Error(err))
+		return nil, status.Error(codes.Internal, "Failed to get unregistered apps")
+	}
+
+	// Integrate all apps cached for each Piped.
+	allApps := make([]*model.ApplicationInfo, 0)
+	for _, as := range pipedToApps {
+		b, ok := as.([]byte)
+		if !ok {
+			return nil, status.Error(codes.Internal, "Unexpected data cached")
+		}
+		dec := gob.NewDecoder(bytes.NewReader(b))
+		var apps []*model.ApplicationInfo
+		if err := dec.Decode(&apps); err != nil {
+			a.logger.Error("failed to decode the unregistered apps", zap.Error(err))
+			return nil, status.Error(codes.Internal, "failed to decode the unregistered apps")
+		}
+		allApps = append(allApps, apps...)
+	}
+	if len(allApps) == 0 {
+		return &webservice.ListUnregisteredApplicationsResponse{}, nil
+	}
+
+	sort.Slice(allApps, func(i, j int) bool {
+		return allApps[i].Path < allApps[j].Path
+	})
+	return &webservice.ListUnregisteredApplicationsResponse{
+		Applications: allApps,
+	}, nil
+}
+
 // TODO: Validate the specified piped to ensure that it belongs to the specified environment.
 func (a *WebAPI) AddApplication(ctx context.Context, req *webservice.AddApplicationRequest) (*webservice.AddApplicationResponse, error) {
 	claims, err := rpcauth.ExtractClaims(ctx)
diff --git a/pkg/app/api/service/webservice/service.proto b/pkg/app/api/service/webservice/service.proto
index b6ad3a96dd..81a215df5d 100644
--- a/pkg/app/api/service/webservice/service.proto
+++ b/pkg/app/api/service/webservice/service.proto
@@ -65,6 +65,7 @@ service WebService {
     rpc SyncApplication(SyncApplicationRequest) returns (SyncApplicationResponse) {}
     rpc GetApplication(GetApplicationRequest) returns (GetApplicationResponse) {}
     rpc GenerateApplicationSealedSecret(GenerateApplicationSealedSecretRequest) returns (GenerateApplicationSealedSecretResponse) {}
+    rpc ListUnregisteredApplications(ListUnregisteredApplicationsRequest) returns (ListUnregisteredApplicationsResponse) {}
 
     // Deployment
     rpc ListDeployments(ListDeploymentsRequest) returns (ListDeploymentsResponse) {}
@@ -219,6 +220,7 @@ message AddEnvironmentRequest {
     string desc = 2;
 }
 
+
 message AddApplicationRequest {
     string name = 1 [(validate.rules).string.min_len = 1];
     string env_id = 2;
@@ -318,6 +320,13 @@ message GenerateApplicationSealedSecretResponse {
     string data = 1 [(validate.rules).string.min_len = 1];
 }
 
+message ListUnregisteredApplicationsRequest {
+}
+
+message ListUnregisteredApplicationsResponse {
+    repeated model.ApplicationInfo applications = 1;
+}
+
 message ListDeploymentsRequest {
     message Options {
         repeated model.DeploymentStatus statuses = 1;
diff --git a/pkg/app/piped/appconfigreporter/appconfigreporter.go b/pkg/app/piped/appconfigreporter/appconfigreporter.go
index e3f9c57d2e..321f87b06a 100644
--- a/pkg/app/piped/appconfigreporter/appconfigreporter.go
+++ b/pkg/app/piped/appconfigreporter/appconfigreporter.go
@@ -370,5 +370,6 @@ func (r *Reporter) readApplicationInfo(repoDir, repoID, appDirRelPath, cfgFilena
 		RepoId:         repoID,
 		Path:           appDirRelPath,
 		ConfigFilename: cfgFilename,
+		PipedId:        r.config.PipedID,
 	}, nil
 }
diff --git a/pkg/app/piped/appconfigreporter/appconfigreporter_test.go b/pkg/app/piped/appconfigreporter/appconfigreporter_test.go
index 29d7ad76eb..87e32afe5d 100644
--- a/pkg/app/piped/appconfigreporter/appconfigreporter_test.go
+++ b/pkg/app/piped/appconfigreporter/appconfigreporter_test.go
@@ -21,6 +21,7 @@ import (
 	"github.com/stretchr/testify/assert"
 	"go.uber.org/zap"
 
+	"github.com/pipe-cd/pipe/pkg/config"
 	"github.com/pipe-cd/pipe/pkg/model"
 )
 
@@ -93,6 +94,7 @@ func TestReporter_findRegisteredApps(t *testing.T) {
 		{
 			name: "app not changed",
 			reporter: &Reporter{
+				config: &config.PipedSpec{PipedID: "piped-1"},
 				applicationLister: &fakeApplicationLister{apps: []*model.Application{
 					{Id: "id-1", Name: "app-1", Labels: map[string]string{"key-1": "value-1"}, GitPath: &model.ApplicationGitPath{Repo: &model.ApplicationGitRepository{Id: "repo-1"}, Path: "app-1", ConfigFilename: ".pipe.yaml"}},
 				}},
@@ -117,6 +119,7 @@ spec:
 		{
 			name: "app changed",
 			reporter: &Reporter{
+				config: &config.PipedSpec{PipedID: "piped-1"},
 				applicationLister: &fakeApplicationLister{apps: []*model.Application{
 					{Id: "id-1", Name: "app-1", Labels: map[string]string{"key-1": "value-1"}, GitPath: &model.ApplicationGitPath{Repo: &model.ApplicationGitRepository{Id: "repo-1"}, Path: "app-1", ConfigFilename: ".pipe.yaml"}},
 				}},
@@ -143,6 +146,7 @@ spec:
 					RepoId:         "repo-1",
 					Path:           "app-1",
 					ConfigFilename: ".pipe.yaml",
+					PipedId:        "piped-1",
 				},
 			},
 			wantErr: false,
@@ -225,6 +229,7 @@ func TestReporter_findUnregisteredApps(t *testing.T) {
 		{
 			name: "valid app config that is unregistered",
 			reporter: &Reporter{
+				config:            &config.PipedSpec{PipedID: "piped-1"},
 				applicationLister: &fakeApplicationLister{},
 				fileSystem: fstest.MapFS{
 					"path/to/repo-1/app-1/.pipe.yaml": &fstest.MapFile{Data: []byte(`
@@ -249,6 +254,7 @@ spec:
 					RepoId:         "repo-1",
 					Path:           "app-1",
 					ConfigFilename: ".pipe.yaml",
+					PipedId:        "piped-1",
 				},
 			},
 			wantErr: false,
@@ -256,6 +262,7 @@ spec:
 		{
 			name: "valid app config that name isn't default",
 			reporter: &Reporter{
+				config:            &config.PipedSpec{PipedID: "piped-1"},
 				applicationLister: &fakeApplicationLister{},
 				fileSystem: fstest.MapFS{
 					"path/to/repo-1/app-1/dev.pipecd.yaml": &fstest.MapFile{Data: []byte(`
@@ -280,6 +287,7 @@ spec:
 					RepoId:         "repo-1",
 					Path:           "app-1",
 					ConfigFilename: "dev.pipecd.yaml",
+					PipedId:        "piped-1",
 				},
 			},
 			wantErr: false,
diff --git a/pkg/cache/rediscache/cache.go b/pkg/cache/rediscache/cache.go
index 01c4b63226..bc7d45b583 100644
--- a/pkg/cache/rediscache/cache.go
+++ b/pkg/cache/rediscache/cache.go
@@ -77,6 +77,7 @@ func (c *RedisCache) Get(k string) (interface{}, error) {
 	return reply, nil
 }
 
+// It is caller's responsibility to encode Go struct.
 func (c *RedisCache) Put(k string, v interface{}) error {
 	conn := c.redis.Get()
 	defer conn.Close()
diff --git a/pkg/cache/rediscache/hashcache.go b/pkg/cache/rediscache/hashcache.go
index 246912583b..9655ddf65d 100644
--- a/pkg/cache/rediscache/hashcache.go
+++ b/pkg/cache/rediscache/hashcache.go
@@ -60,6 +60,8 @@ func (r *RedisHashCache) Get(k string) (interface{}, error) {
 // check if the TTL for hashkey (not the key k), in case the TTL for
 // hashkey is not yet existed or unset, EXPIRE will be called and set
 // TTL time for the whole hashkey.
+//
+// It is caller's responsibility to encode Go struct.
 func (r *RedisHashCache) Put(k string, v interface{}) error {
 	conn := r.redis.Get()
 	defer conn.Close()
diff --git a/pkg/model/common.proto b/pkg/model/common.proto
index 1d4b2d65c2..54d109d690 100644
--- a/pkg/model/common.proto
+++ b/pkg/model/common.proto
@@ -68,4 +68,5 @@ message ApplicationInfo {
     string path = 6 [(validate.rules).string.pattern = "^[^/].+$"];
     // This field is not allowed to be changed.
     string config_filename = 7;
+    string piped_id = 8 [(validate.rules).string.min_len = 1];
 }