diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index 6d0d741..edcc1b2 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -62,12 +62,12 @@ jobs:
     - name: Build test dependencies (healthworker)
       run: go build ./testdata/healthworker/...
 
-    - name: Run cgroup integration tests (as root)
+    - name: Run cgroup + namespace integration tests (as root)
       if: env.CGROUP_AVAILABLE == 'true'
       run: |
         sudo --preserve-env=PATH,GOPATH,GOCACHE,HOME \
           env HERD_CGROUP_TEST=1 \
-          $(which go) test -v -run TestSandbox -timeout 60s ./...
+          $(which go) test -v -run 'TestSandbox|TestNamespace' -timeout 90s ./...
       env:
         GOPATH: ${{ env.GOPATH }}
         GOCACHE: ${{ env.GOCACHE }}
diff --git a/factory_cgroup_test.go b/factory_cgroup_test.go
index 384c6ec..829853b 100644
--- a/factory_cgroup_test.go
+++ b/factory_cgroup_test.go
@@ -25,6 +25,13 @@ func TestNewProcessFactory_DefaultMemoryCPUUnlimited(t *testing.T) {
 	}
 }
 
+func TestNewProcessFactory_DefaultNamespaceFlags(t *testing.T) {
+	f := NewProcessFactory("./fake-binary")
+	if f.namespaceCloneFlags != defaultNamespaceCloneFlags() {
+		t.Errorf("expected default namespaceCloneFlags=%d, got %d", defaultNamespaceCloneFlags(), f.namespaceCloneFlags)
+	}
+}
+
 func TestWithMemoryLimit_StoresBytes(t *testing.T) {
 	const limit = 512 * 1024 * 1024 // 512 MB
 	f := NewProcessFactory("./fake-binary").WithMemoryLimit(limit)
diff --git a/process_worker_factory.go b/process_worker_factory.go
index 38eae8b..c08e04b 100644
--- a/process_worker_factory.go
+++ b/process_worker_factory.go
@@ -172,6 +172,7 @@ type ProcessFactory struct {
 	startTimeout          time.Duration // maximum time to wait for the first successful health check
 	startHealthCheckDelay time.Duration // delay the health check for the first time.
 	enableSandbox         bool          // true by default for isolation
+	namespaceCloneFlags   uintptr       // Linux namespaces to enable for sandboxed workers
 	cgroupMemory          int64         // bytes; 0 means unlimited
 	cgroupCPU             int64         // quota in micros per 100ms period; 0 means unlimited
 	cgroupPIDs            int64         // max pids; -1 means unlimited
@@ -193,6 +194,7 @@ func NewProcessFactory(binary string, args ...string) *ProcessFactory {
 		startTimeout:          30 * time.Second,
 		startHealthCheckDelay: 1 * time.Second,
 		enableSandbox:         true,
+		namespaceCloneFlags:   defaultNamespaceCloneFlags(),
 		cgroupPIDs:            100,
 	}
 }
@@ -331,6 +333,7 @@ func (f *ProcessFactory) Spawn(ctx context.Context) (Worker[*http.Client], error
 			memoryMaxBytes: f.cgroupMemory,
 			cpuMaxMicros:   f.cgroupCPU,
 			pidsMax:        f.cgroupPIDs,
+			cloneFlags:     f.namespaceCloneFlags,
 		})
 		if err != nil {
 			return nil, fmt.Errorf("herd: ProcessFactory: failed to apply sandbox: %w", err)
diff --git a/sandbox.go b/sandbox.go
index ff3f9ed..d961323 100644
--- a/sandbox.go
+++ b/sandbox.go
@@ -6,6 +6,7 @@ type sandboxConfig struct {
 	memoryMaxBytes int64
 	cpuMaxMicros   int64
 	pidsMax        int64
+	cloneFlags     uintptr
 }
 
 // sandboxHandle owns post-start and cleanup hooks for sandbox resources.
diff --git a/sandbox_integration_test.go b/sandbox_integration_test.go
index f04493c..93876bf 100644
--- a/sandbox_integration_test.go
+++ b/sandbox_integration_test.go
@@ -19,6 +19,7 @@ import (
 	"os"
 	"os/exec"
 	"path/filepath"
+	"strconv"
 	"strings"
 	"testing"
 	"time"
@@ -240,3 +241,65 @@ func TestSandbox_MemoryLimitFileWritten(t *testing.T) {
 	t.Logf("memory limits confirmed: max=%s swap=%s",
 		strings.TrimSpace(string(memMax)), strings.TrimSpace(string(swapMax)))
 }
+
+func TestNamespace_PIDIsolation(t *testing.T) {
+	requireCgroupIntegration(t)
+
+	bin := buildHealthWorker(t)
+
+	factory := NewProcessFactory(bin).
+		WithHealthPath("/health").
+		WithStartTimeout(10 * time.Second).
+		WithStartHealthCheckDelay(100 * time.Millisecond)
+
+	ctx, cancel := context.WithTimeout(context.Background(), 20*time.Second)
+	defer cancel()
+
+	worker, err := factory.Spawn(ctx)
+	if err != nil {
+		t.Fatalf("Spawn: %v", err)
+	}
+	defer worker.Close()
+
+	pw, ok := worker.(*processWorker)
+	if !ok {
+		t.Fatal("expected *processWorker from Spawn")
+	}
+	pw.mu.Lock()
+	hostPID := pw.cmd.Process.Pid
+	pw.mu.Unlock()
+
+	statusFile := fmt.Sprintf("/proc/%d/status", hostPID)
+	status, err := os.ReadFile(statusFile)
+	if err != nil {
+		t.Fatalf("read %s: %v", statusFile, err)
+	}
+
+	insidePID, ok := parseInnermostNSpid(string(status))
+	if !ok {
+		t.Fatalf("NSpid line not found in %s:\n%s", statusFile, string(status))
+	}
+	if insidePID != 1 {
+		t.Fatalf("expected worker to be PID 1 inside its namespace, got %d", insidePID)
+	}
+	t.Logf("NSpid verified: host pid=%d, namespace pid=%d", hostPID, insidePID)
+}
+
+func parseInnermostNSpid(status string) (int, bool) {
+	for _, line := range strings.Split(status, "\n") {
+		if !strings.HasPrefix(line, "NSpid:") {
+			continue
+		}
+		fields := strings.Fields(strings.TrimPrefix(line, "NSpid:"))
+		if len(fields) == 0 {
+			return 0, false
+		}
+		last := fields[len(fields)-1]
+		pid, err := strconv.Atoi(last)
+		if err != nil {
+			return 0, false
+		}
+		return pid, true
+	}
+	return 0, false
+}
diff --git a/sandbox_linux.go b/sandbox_linux.go
index 3694edd..e2a6a50 100644
--- a/sandbox_linux.go
+++ b/sandbox_linux.go
@@ -112,6 +112,9 @@ func applySandboxFlags(cmd *exec.Cmd, workerID string, cfg sandboxConfig) (sandb
 	if sys == nil {
 		sys = &syscall.SysProcAttr{}
 	}
+	if cfg.cloneFlags != 0 {
+		sys.Cloneflags |= cfg.cloneFlags
+	}
 	sys.CgroupFD = int(dir.Fd())
 	sys.UseCgroupFD = true
 	cmd.SysProcAttr = sys
@@ -119,6 +122,10 @@ func applySandboxFlags(cmd *exec.Cmd, workerID string, cfg sandboxConfig) (sandb
 	return &cgroupHandle{path: cgroupPath, fd: dir}, nil
 }
 
+func defaultNamespaceCloneFlags() uintptr {
+	return uintptr(syscall.CLONE_NEWPID | syscall.CLONE_NEWNS | syscall.CLONE_NEWIPC)
+}
+
 func writeCgroupFile(cgroupPath, filename, value string) error {
 	path := filepath.Join(cgroupPath, filename)
 	return os.WriteFile(path, []byte(value), 0o644)
diff --git a/sandbox_linux_test.go b/sandbox_linux_test.go
index 5e4f74e..e977f2e 100644
--- a/sandbox_linux_test.go
+++ b/sandbox_linux_test.go
@@ -12,6 +12,7 @@ import (
 	"os/exec"
 	"path/filepath"
 	"strings"
+	"syscall"
 	"testing"
 )
 
@@ -163,6 +164,32 @@ func TestApplySandboxFlags_SysProcAttrWired(t *testing.T) {
 	}
 }
 
+func TestApplySandboxFlags_CloneFlagsMergedWithCgroup(t *testing.T) {
+	withTempCgroupRoot(t)
+	cmd := newFakeCmd()
+
+	flags := uintptr(syscall.CLONE_NEWPID | syscall.CLONE_NEWNS | syscall.CLONE_NEWIPC)
+	h, err := applySandboxFlags(cmd, "worker-ns", sandboxConfig{cloneFlags: flags})
+	if err != nil {
+		t.Fatalf("unexpected error: %v", err)
+	}
+	if h == nil {
+		t.Fatal("expected non-nil handle")
+	}
+	if cmd.SysProcAttr == nil {
+		t.Fatal("SysProcAttr should be set after applySandboxFlags")
+	}
+	if cmd.SysProcAttr.Cloneflags&flags != flags {
+		t.Errorf("expected Cloneflags to include %#x, got %#x", flags, cmd.SysProcAttr.Cloneflags)
+	}
+	if !cmd.SysProcAttr.UseCgroupFD {
+		t.Error("UseCgroupFD should remain true after namespace merge")
+	}
+	if cmd.SysProcAttr.CgroupFD <= 0 {
+		t.Errorf("CgroupFD should remain set after namespace merge, got %d", cmd.SysProcAttr.CgroupFD)
+	}
+}
+
 func TestApplySandboxFlags_SoftFailOnBadRoot(t *testing.T) {
 	// Point to a path that cannot be created (inside /proc which is read-only).
 	old := activeCgroupRoot
diff --git a/sandbox_unsupported.go b/sandbox_unsupported.go
index 908aa65..6fbf85d 100644
--- a/sandbox_unsupported.go
+++ b/sandbox_unsupported.go
@@ -27,3 +27,7 @@ var ErrSandboxUnsupported = errors.New(
 func applySandboxFlags(cmd *exec.Cmd, workerID string, cfg sandboxConfig) (sandboxHandle, error) {
 	return nil, ErrSandboxUnsupported
 }
+
+func defaultNamespaceCloneFlags() uintptr {
+	return 0
+}