diff --git a/.github/workflows/example-14.yml b/.github/workflows/example-14.yml
new file mode 100644
index 00000000..f73d9a95
--- /dev/null
+++ b/.github/workflows/example-14.yml
@@ -0,0 +1,81 @@
+name: "Example 14: Remove defaults channel"
+
+on:
+  pull_request:
+    branches:
+      - "*"
+  push:
+    branches:
+      - "develop"
+      - "main"
+      - "master"
+  schedule:
+    # Note that cronjobs run on master/main by default
+    - cron: "0 0 * * *"
+
+concurrency:
+  group:
+    ${{ github.workflow }}-${{ github.event.pull_request.number || github.sha }}
+  cancel-in-progress: true
+
+jobs:
+  example-14:
+    # prevent cronjobs from running on forks
+    if:
+      (github.event_name == 'schedule' && github.repository ==
+      'conda-incubator/setup-miniconda') || (github.event_name != 'schedule')
+    name:
+      Ex14 (${{ matrix.os }}, remove=${{ matrix.conda-remove-defaults }},
+      channels=${{ matrix.channels }})
+    runs-on: ${{ matrix.os }}
+    strategy:
+      fail-fast: false
+      matrix:
+        os: ["ubuntu-latest", "macos-latest", "windows-latest"]
+        conda-remove-defaults: ["true", "false"]
+        channels:
+          [
+            "conda-forge",
+            "conda-forge,nodefaults",
+            "conda-forge,defaults,nodefaults",
+          ]
+    timeout-minutes: 30
+    steps:
+      - uses: actions/checkout@v4
+      - uses: ./
+        id: setup-miniconda
+        continue-on-error: true
+        with:
+          channels: ${{ matrix.channels }}
+          conda-remove-defaults: ${{ matrix.conda-remove-defaults }}
+          python-version: "3.12"
+      - shell: bash -el {0}
+        run: |
+          cat <<EOF | python
+          import json
+          import os
+          from subprocess import check_output
+
+          out = check_output(
+            [
+              os.environ["CONDA_EXE"],
+              "config",
+              "--show",
+              "channels",
+              "--json",
+            ]
+          )
+          channels = json.loads(out)["channels"]
+          input_channels = "${{ matrix.channels }}".split(",")
+          remove_defaults = "${{ matrix.conda-remove-defaults }}" == "true"
+
+          print("Input channels:", input_channels)
+          print("Input remove-defaults:", remove_defaults)
+          print("Computed channels:", channels)
+
+          assert "nodefaults" not in channels
+          if "defaults" in input_channels:
+            assert "defaults" in channels
+          elif "nodefaults" in input_channels or remove_defaults:
+            assert "defaults" not in channels
+          EOF
diff --git a/.github/workflows/example-6.yml b/.github/workflows/example-6.yml
index 8bd0b677..1644c604 100644
--- a/.github/workflows/example-6.yml
+++ b/.github/workflows/example-6.yml
@@ -49,6 +49,13 @@ jobs:
       - uses: ./
         with:
           miniforge-variant: Miniforge3
+          # Use an old Miniforge (without conda-libmamba-solver) to allow updates to mamba v2.
+          # Set the solver to classic for those too. This can be removed when
+          # conda-libmamba-solver is released with libmamba v2 compatibility.
+          miniforge-version:
+            ${{ matrix.mamba-version == '2' && '23.1.0-0' || 'latest' }}
+          conda-solver:
+            ${{ matrix.mamba-version == '2' && 'classic' || 'libmamba' }}
           python-version: "3.11"
           mamba-version: ${{ matrix.mamba-version }}
           channels: conda-forge,nodefaults
diff --git a/README.md b/README.md
index 2150f7ec..ff6909a1 100644
--- a/README.md
+++ b/README.md
@@ -59,6 +59,7 @@ possibility of automatically activating the `test` environment on all shells.
 | [Caching packages](#caching-packages)                              | [![Caching Example Status][caching-badge]][caching]             |
 | [Caching environments](#caching-environments)                      | [![Caching Env Example Status][caching-env-badge]][caching-env] |
 | [Apple Silicon](#example-13-apple-silicon)                         | [![Apple Silicon][ex13-badge]][ex13]                            |
+| [Remove defaults](#example-14-conda-remove-defaults)               | [![Remove defaults][ex14-badge]][ex14]                          |
 
 [ex1]:
   https://github.com/conda-incubator/setup-miniconda/actions/workflows/example-1.yml
@@ -112,6 +113,10 @@ possibility of automatically activating the `test` environment on all shells.
   https://github.com/conda-incubator/setup-miniconda/actions/workflows/example-13.yml
 [ex13-badge]:
   https://github.com/conda-incubator/setup-miniconda/actions/workflows/example-13.yml/badge.svg?branch=main
+[ex14]:
+  https://github.com/conda-incubator/setup-miniconda/actions/workflows/example-14.yml
+[ex14-badge]:
+  https://github.com/conda-incubator/setup-miniconda/actions/workflows/example-14.yml/badge.svg?branch=main
 
 ## Other Workflows
 
@@ -598,6 +603,35 @@ jobs:
           python -c "import platform; assert platform.machine() == 'arm64', platform.machine()"
 ```
 
+### Example 14: Remove `defaults` channel
+
+Workaround for this bug:
+[conda#12356](https://github.com/conda/conda/issues/12356).
+
+```yaml
+jobs:
+  example-13:
+    name: Ex14 (os=${{ matrix.os }})
+    runs-on: ${{ matrix.os }}
+    strategy:
+      fail-fast: false
+      matrix:
+        os: ["ubuntu-latest"]
+    steps:
+      - uses: actions/checkout@v4
+      - uses: ./
+        id: setup-miniconda
+        continue-on-error: true
+        with:
+          miniforge-version: latest
+          channels: conda-forge
+          conda-remove-defaults: "true"
+      - name: Check config
+        shell: bash -el {0}
+        run: |
+          conda config --show-sources
+```
+
 ## Caching
 
 ### Caching packages
diff --git a/action.yml b/action.yml
index 9e1699d7..82079de5 100644
--- a/action.yml
+++ b/action.yml
@@ -193,6 +193,13 @@ inputs:
       for more information."
     required: false
     default: ""
+  conda-remove-defaults:
+    description:
+      "Postprocess channels list to remove 'defaults' if it was added implicitly
+      to the 'channels' setting. Will default to 'true' to align with related
+      changes in conda 25.3.0."
+    required: false
+    default: ""
   show-channel-urls:
     description:
       'Conda configuration. Show channel URLs when displaying what is going to
diff --git a/dist/setup/index.js b/dist/setup/index.js
index 500406ae..b574b5a1 100644
--- a/dist/setup/index.js
+++ b/dist/setup/index.js
@@ -47349,12 +47349,37 @@ function applyCondaConfiguration(inputs, options) {
         if (!channels.length && ((_c = (_b = (_a = options.envSpec) === null || _a === void 0 ? void 0 : _a.yaml) === null || _b === void 0 ? void 0 : _b.channels) === null || _c === void 0 ? void 0 : _c.length)) {
             channels = options.envSpec.yaml.channels;
         }
+        // This can be enabled via conda-remove-defaults and channels = nodefaults
+        let removeDefaults = inputs.condaRemoveDefaults === "true";
         // LIFO: reverse order to preserve higher priority as listed in the option
         // .slice ensures working against a copy
         for (const channel of channels.slice().reverse()) {
+            if (channel === "nodefaults") {
+                core.warning("'nodefaults' channel detected: will remove 'defaults' if added implicitly. " +
+                    "In the future, 'nodefaults' as a way of removing 'defaults' won't be supported. " +
+                    "Please set 'conda-remove-defaults' = 'true' to remove this warning.");
+                removeDefaults = true;
+                continue;
+            }
             core.info(`Adding channel '${channel}'`);
             yield condaCommand(["config", "--add", "channels", channel], options);
         }
+        if (!channels.includes("defaults")) {
+            if (removeDefaults) {
+                core.info("Removing implicitly added 'defaults' channel");
+                try {
+                    yield condaCommand(["config", "--remove", "channels", "defaults"], options);
+                }
+                catch (err) {
+                    core.info("Removing defaults raised an error -- it was probably not present.");
+                }
+            }
+            else {
+                core.warning("The 'defaults' channel might have been added implicitly. " +
+                    "If this is intentional, add 'defaults' to the 'channels' list. " +
+                    "Otherwise, consider setting 'conda-remove-defaults' to 'true'.");
+            }
+        }
         // All other options are just passed as their string representations
         for (const [key, value] of configEntries) {
             if (value.trim().length === 0 || key === "channels") {
@@ -48188,6 +48213,7 @@ function parseInputs() {
             minicondaVersion: core.getInput("miniconda-version"),
             miniforgeVariant: core.getInput("miniforge-variant"),
             miniforgeVersion: core.getInput("miniforge-version"),
+            condaRemoveDefaults: core.getInput("conda-remove-defaults"),
             pythonVersion: core.getInput("python-version"),
             removeProfiles: core.getInput("remove-profiles"),
             condaConfig: Object.freeze({
diff --git a/src/conda.ts b/src/conda.ts
index 2359d56e..2c46db36 100644
--- a/src/conda.ts
+++ b/src/conda.ts
@@ -159,13 +159,47 @@ export async function applyCondaConfiguration(
     channels = options.envSpec.yaml.channels;
   }
 
+  // This can be enabled via conda-remove-defaults and channels = nodefaults
+  let removeDefaults: boolean = inputs.condaRemoveDefaults === "true";
+
   // LIFO: reverse order to preserve higher priority as listed in the option
   // .slice ensures working against a copy
   for (const channel of channels.slice().reverse()) {
+    if (channel === "nodefaults") {
+      core.warning(
+        "'nodefaults' channel detected: will remove 'defaults' if added implicitly. " +
+          "In the future, 'nodefaults' as a way of removing 'defaults' won't be supported. " +
+          "Please set 'conda-remove-defaults' = 'true' to remove this warning.",
+      );
+      removeDefaults = true;
+      continue;
+    }
     core.info(`Adding channel '${channel}'`);
     await condaCommand(["config", "--add", "channels", channel], options);
   }
 
+  if (!channels.includes("defaults")) {
+    if (removeDefaults) {
+      core.info("Removing implicitly added 'defaults' channel");
+      try {
+        await condaCommand(
+          ["config", "--remove", "channels", "defaults"],
+          options,
+        );
+      } catch (err) {
+        core.info(
+          "Removing defaults raised an error -- it was probably not present.",
+        );
+      }
+    } else {
+      core.warning(
+        "The 'defaults' channel might have been added implicitly. " +
+          "If this is intentional, add 'defaults' to the 'channels' list. " +
+          "Otherwise, consider setting 'conda-remove-defaults' to 'true'.",
+      );
+    }
+  }
+
   // All other options are just passed as their string representations
   for (const [key, value] of configEntries) {
     if (value.trim().length === 0 || key === "channels") {
diff --git a/src/input.ts b/src/input.ts
index 65f00e1c..b75c7633 100644
--- a/src/input.ts
+++ b/src/input.ts
@@ -92,6 +92,7 @@ export async function parseInputs(): Promise<types.IActionInputs> {
     minicondaVersion: core.getInput("miniconda-version"),
     miniforgeVariant: core.getInput("miniforge-variant"),
     miniforgeVersion: core.getInput("miniforge-version"),
+    condaRemoveDefaults: core.getInput("conda-remove-defaults"),
     pythonVersion: core.getInput("python-version"),
     removeProfiles: core.getInput("remove-profiles"),
     condaConfig: Object.freeze({
diff --git a/src/types.ts b/src/types.ts
index 814451ce..d505319d 100644
--- a/src/types.ts
+++ b/src/types.ts
@@ -84,6 +84,7 @@ export interface IActionInputs {
   readonly minicondaVersion: string;
   readonly miniforgeVariant: string;
   readonly miniforgeVersion: string;
+  readonly condaRemoveDefaults: string;
   readonly pythonVersion: string;
   readonly removeProfiles: string;
   readonly useMamba: string;