diff --git a/internal/chezmoi/chezmoi.go b/internal/chezmoi/chezmoi.go
index 7fb2f7edaf0..75b55c6b177 100644
--- a/internal/chezmoi/chezmoi.go
+++ b/internal/chezmoi/chezmoi.go
@@ -131,6 +131,11 @@ var FileModeTypeNames = map[fs.FileMode]string{
 	fs.ModeCharDevice: "char device",
 }
 
+// A TextConvFunc converts the contents of a file into a more human-readable
+// form. It returns the converted data, whether any conversion occurred, and any
+// error.
+type TextConvFunc func(string, []byte) ([]byte, bool, error)
+
 // FQDNHostname returns the FQDN hostname.
 func FQDNHostname(fileSystem vfs.FS) (string, error) {
 	// First, try os.Hostname. If it returns something that looks like a FQDN
diff --git a/internal/chezmoi/externaldiffsystem.go b/internal/chezmoi/externaldiffsystem.go
index 9e92b74f79d..8f7410f6f12 100644
--- a/internal/chezmoi/externaldiffsystem.go
+++ b/internal/chezmoi/externaldiffsystem.go
@@ -27,6 +27,7 @@ type ExternalDiffSystem struct {
 	filter         *EntryTypeFilter
 	reverse        bool
 	scriptContents bool
+	textConvFunc   TextConvFunc
 }
 
 // ExternalDiffSystemOptions are options for NewExternalDiffSystem.
@@ -34,6 +35,7 @@ type ExternalDiffSystemOptions struct {
 	Filter         *EntryTypeFilter
 	Reverse        bool
 	ScriptContents bool
+	TextConvFunc   TextConvFunc
 }
 
 // NewExternalDiffSystem creates a new ExternalDiffSystem.
@@ -52,6 +54,7 @@ func NewExternalDiffSystem(
 		filter:         options.Filter,
 		reverse:        options.Reverse,
 		scriptContents: options.ScriptContents,
+		textConvFunc:   options.TextConvFunc,
 	}
 }
 
@@ -222,34 +225,68 @@ func (s *ExternalDiffSystem) UnderlyingFS() vfs.FS {
 // WriteFile implements System.WriteFile.
 func (s *ExternalDiffSystem) WriteFile(filename AbsPath, data []byte, perm fs.FileMode) error {
 	if s.filter.IncludeEntryTypeBits(EntryTypeFiles) {
+		targetRelPath, err := filename.TrimDirPrefix(s.destDirAbsPath)
+		if err != nil {
+			return err
+		}
+		tempDirAbsPath, err := s.tempDir()
+		if err != nil {
+			return err
+		}
+
 		// If filename does not exist, replace it with /dev/null to avoid
 		// passing the name of a non-existent file to the external diff command.
-		destAbsPath := filename
-		switch _, err := os.Stat(destAbsPath.String()); {
+		// Otherwise, if the file exists and a textconv filter is configured,
+		// run the filter and update fromAbsPath to point to the converted data.
+		fromAbsPath := filename
+		switch fileInfo, err := os.Lstat(fromAbsPath.String()); {
 		case errors.Is(err, fs.ErrNotExist):
-			destAbsPath = devNullAbsPath
+			fromAbsPath = devNullAbsPath
 		case err != nil:
 			return err
+		case s.textConvFunc != nil:
+			// Maybe convert the from data with textconv.
+			fromData, err := os.ReadFile(fromAbsPath.String())
+			if err != nil {
+				return err
+			}
+			switch convertedFromData, converted, err := s.textConvFunc(fromAbsPath.String(), fromData); {
+			case err != nil:
+				return err
+			case converted:
+				tempFromAbsPath := tempDirAbsPath.Join(NewRelPath("a"), targetRelPath)
+				if err := os.MkdirAll(tempFromAbsPath.Dir().String(), 0o700); err != nil {
+					return err
+				}
+				if err := os.WriteFile(tempFromAbsPath.String(), convertedFromData, fileInfo.Mode().Perm()); err != nil {
+					return err
+				}
+				fromAbsPath = tempFromAbsPath
+			}
 		}
 
 		// Write the target contents to a file in a temporary directory.
-		targetRelPath, err := filename.TrimDirPrefix(s.destDirAbsPath)
-		if err != nil {
-			return err
-		}
-		tempDirAbsPath, err := s.tempDir()
-		if err != nil {
-			return err
+		toAbsPath := tempDirAbsPath.Join(targetRelPath)
+		toData := data
+		if s.textConvFunc != nil {
+			// Maybe convert the to data with textconv.
+			switch convertedToData, converted, err := s.textConvFunc(filename.String(), toData); {
+			case err != nil:
+				return err
+			case converted:
+				toAbsPath = tempDirAbsPath.Join(NewRelPath("b"), targetRelPath)
+				toData = convertedToData
+			}
 		}
-		targetAbsPath := tempDirAbsPath.Join(targetRelPath)
-		if err := os.MkdirAll(targetAbsPath.Dir().String(), 0o700); err != nil {
+		if err := os.MkdirAll(toAbsPath.Dir().String(), 0o700); err != nil {
 			return err
 		}
-		if err := os.WriteFile(targetAbsPath.String(), data, perm); err != nil {
+		if err := os.WriteFile(toAbsPath.String(), toData, perm); err != nil {
 			return err
 		}
 
-		if err := s.runDiffCommand(destAbsPath, targetAbsPath); err != nil {
+		// Run the external diff command.
+		if err := s.runDiffCommand(fromAbsPath, toAbsPath); err != nil {
 			return err
 		}
 	}
diff --git a/internal/chezmoi/gitdiffsystem.go b/internal/chezmoi/gitdiffsystem.go
index 7189ac80e93..047af3ee835 100644
--- a/internal/chezmoi/gitdiffsystem.go
+++ b/internal/chezmoi/gitdiffsystem.go
@@ -14,9 +14,6 @@ import (
 	vfs "github.com/twpayne/go-vfs/v5"
 )
 
-// A TextConvFunc converts the contents of a file into a more human-readable form.
-type TextConvFunc func(string, []byte) ([]byte, error)
-
 // A GitDiffSystem wraps a System and logs all of the actions executed as a git
 // diff.
 type GitDiffSystem struct {
@@ -278,7 +275,7 @@ func (s *GitDiffSystem) encodeDiff(absPath AbsPath, toData []byte, toMode fs.Fil
 			return err
 		}
 		if s.textConvFunc != nil {
-			fromData, err = s.textConvFunc(absPath.String(), fromData)
+			fromData, _, err = s.textConvFunc(absPath.String(), fromData)
 			if err != nil {
 				return err
 			}
@@ -297,7 +294,7 @@ func (s *GitDiffSystem) encodeDiff(absPath AbsPath, toData []byte, toMode fs.Fil
 
 	if s.textConvFunc != nil {
 		var err error
-		toData, err = s.textConvFunc(absPath.String(), toData)
+		toData, _, err = s.textConvFunc(absPath.String(), toData)
 		if err != nil {
 			return err
 		}
diff --git a/internal/cmd/config.go b/internal/cmd/config.go
index eda605af996..1302a281c4c 100644
--- a/internal/cmd/config.go
+++ b/internal/cmd/config.go
@@ -1228,14 +1228,14 @@ func (c *Config) diffFile(
 	}
 	if fromMode.IsRegular() {
 		var err error
-		fromData, err = c.TextConv.convert(path.String(), fromData)
+		fromData, _, err = c.TextConv.convert(path.String(), fromData)
 		if err != nil {
 			return err
 		}
 	}
 	if toMode.IsRegular() {
 		var err error
-		toData, err = c.TextConv.convert(path.String(), toData)
+		toData, _, err = c.TextConv.convert(path.String(), toData)
 		if err != nil {
 			return err
 		}
@@ -1817,6 +1817,7 @@ func (c *Config) newDiffSystem(s chezmoi.System, w io.Writer, dirAbsPath chezmoi
 		Filter:         chezmoi.NewEntryTypeFilter(c.Diff.include.Bits(), c.Diff.Exclude.Bits()),
 		Reverse:        c.Diff.Reverse,
 		ScriptContents: c.Diff.ScriptContents,
+		TextConvFunc:   c.TextConv.convert,
 	}
 	return chezmoi.NewExternalDiffSystem(s, c.Diff.Command, c.Diff.Args, c.DestDirAbsPath, options)
 }
diff --git a/internal/cmd/testdata/scripts/externaldiff.txtar b/internal/cmd/testdata/scripts/externaldiff.txtar
index fe6b83163f7..3c743aeec0d 100644
--- a/internal/cmd/testdata/scripts/externaldiff.txtar
+++ b/internal/cmd/testdata/scripts/externaldiff.txtar
@@ -1,5 +1,7 @@
 [windows] skip 'UNIX only'
 
+chmod 755 bin/external-diff
+
 # test that chezmoi diff invokes the external diff command for scripts
 exec chezmoi diff
 stdout '# contents of script'
@@ -20,6 +22,26 @@ chhome home3/user
 exec chezmoi diff
 stdout ^/dev/null\s${WORK@R}/.*/\.dir$
 
+chhome home4/user
+
+# test that chezmoi diff uses textconv when an external diff tool is used
+exec chezmoi diff
+cmp stdout golden/external-diff
+
+-- bin/external-diff --
+#!/bin/sh
+
+echo old/$(basename $1):
+cat $1
+echo
+echo new/$(basename $2):
+cat $2
+-- golden/external-diff --
+old/file.txt:
+# OLD CONTENTS OF .DIR/FILE.TXT
+
+new/file.txt:
+# NEW CONTENTS OF .DIR/FILE.TXT
 -- home/user/.config/chezmoi/chezmoi.toml --
 [diff]
     command = "cat"
@@ -34,3 +56,16 @@ diff:
 [diff]
     command = "echo"
 -- home3/user/.local/share/chezmoi/dot_dir/.keep --
+-- home4/user/.config/chezmoi/chezmoi.yaml --
+diff:
+  command: external-diff
+textconv:
+- pattern: '**/*.txt'
+  command: tr
+  args:
+  - a-z
+  - A-Z
+-- home4/user/.dir/file.txt --
+# old contents of .dir/file.txt
+-- home4/user/.local/share/chezmoi/dot_dir/file.txt --
+# new contents of .dir/file.txt
diff --git a/internal/cmd/textconv.go b/internal/cmd/textconv.go
index 2d1a102fe05..a7db3c67bba 100644
--- a/internal/cmd/textconv.go
+++ b/internal/cmd/textconv.go
@@ -19,12 +19,12 @@ type textConvElement struct {
 
 type textConv []*textConvElement
 
-func (t textConv) convert(path string, data []byte) ([]byte, error) {
+func (t textConv) convert(path string, data []byte) ([]byte, bool, error) {
 	var longestPatternElement *textConvElement
 	for _, command := range t {
 		ok, err := doublestar.Match(command.Pattern, path)
 		if err != nil {
-			return nil, err
+			return nil, false, err
 		}
 		if !ok {
 			continue
@@ -34,11 +34,15 @@ func (t textConv) convert(path string, data []byte) ([]byte, error) {
 		}
 	}
 	if longestPatternElement == nil {
-		return data, nil
+		return data, false, nil
 	}
 
 	cmd := exec.Command(longestPatternElement.Command, longestPatternElement.Args...)
 	cmd.Stdin = bytes.NewReader(data)
 	cmd.Stderr = os.Stderr
-	return chezmoilog.LogCmdOutput(slog.Default(), cmd)
+	convertedData, err := chezmoilog.LogCmdOutput(slog.Default(), cmd)
+	if err != nil {
+		return nil, false, err
+	}
+	return convertedData, true, nil
 }