diff --git a/internal/db/io_markers.go b/internal/db/io_markers.go
new file mode 100644
index 0000000..afda0ce
--- /dev/null
+++ b/internal/db/io_markers.go
@@ -0,0 +1,180 @@
+package db
+
+import (
+	"database/sql"
+	"errors"
+	"fmt"
+	"strings"
+)
+
+// IOMarker is a wall-outlet terminator inside a project. Mostly Power
+// by convention; the schema doesn't enforce it.
+type IOMarker struct {
+	ID            int64   `json:"id"`
+	ProjectID     int64   `json:"project_id"`
+	FrameID       *int64  `json:"frame_id"`
+	Label         string  `json:"label"`
+	X             float64 `json:"x"`
+	Y             float64 `json:"y"`
+	ExcalidrawID  *string `json:"excalidraw_id,omitempty"`
+	CreatedAt     string  `json:"created_at"`
+	UpdatedAt     string  `json:"updated_at"`
+}
+
+// IOMarkerCreate is the create-shape.
+type IOMarkerCreate struct {
+	FrameID *int64
+	Label   string
+	X       float64
+	Y       float64
+}
+
+// IOMarkerUpdate is the partial-update shape. project_id deliberately not
+// settable; frame_id uses the same tri-state shape as DeviceUpdate.FrameID.
+type IOMarkerUpdate struct {
+	Label   *string
+	FrameID FrameRef
+	X       *float64
+	Y       *float64
+}
+
+// CreateIOMarker inserts a new IO marker. If frame_id is set, it must
+// reference a frame in the same project.
+func (s *Store) CreateIOMarker(projectID int64, m IOMarkerCreate) (*IOMarker, error) {
+	label := strings.TrimSpace(m.Label)
+	if label == "" {
+		label = "IO"
+	}
+	if _, err := s.GetProject(projectID); err != nil {
+		return nil, err
+	}
+	if m.FrameID != nil {
+		if _, err := s.GetFrame(projectID, *m.FrameID); err != nil {
+			if errors.Is(err, ErrNotFound) {
+				return nil, fmt.Errorf("%w: frame_id %d not in project %d", ErrInvalidInput, *m.FrameID, projectID)
+			}
+			return nil, err
+		}
+	}
+	res, err := s.db.Exec(
+		`INSERT INTO io_markers (project_id, frame_id, label, x, y)
+		 VALUES (?, ?, ?, ?, ?)`,
+		projectID, nullableInt64(m.FrameID), label, m.X, m.Y,
+	)
+	if err != nil {
+		return nil, mapWriteErr(err)
+	}
+	id, _ := res.LastInsertId()
+	return s.GetIOMarker(projectID, id)
+}
+
+// GetIOMarker loads an IO marker, project-scoped.
+func (s *Store) GetIOMarker(projectID, id int64) (*IOMarker, error) {
+	var m IOMarker
+	var frame sql.NullInt64
+	var ex sql.NullString
+	err := s.db.QueryRow(
+		`SELECT id, project_id, frame_id, label, x, y, excalidraw_id, created_at, updated_at
+		 FROM io_markers WHERE id = ? AND project_id = ?`, id, projectID,
+	).Scan(&m.ID, &m.ProjectID, &frame, &m.Label, &m.X, &m.Y, &ex, &m.CreatedAt, &m.UpdatedAt)
+	if errors.Is(err, sql.ErrNoRows) {
+		return nil, ErrNotFound
+	}
+	if err != nil {
+		return nil, err
+	}
+	if frame.Valid {
+		v := frame.Int64
+		m.FrameID = &v
+	}
+	if ex.Valid {
+		m.ExcalidrawID = &ex.String
+	}
+	return &m, nil
+}
+
+// ListIOMarkers returns every IO marker in a project, ordered by creation.
+func (s *Store) ListIOMarkers(projectID int64) ([]IOMarker, error) {
+	rows, err := s.db.Query(
+		`SELECT id, project_id, frame_id, label, x, y, excalidraw_id, created_at, updated_at
+		 FROM io_markers WHERE project_id = ? ORDER BY created_at, id`, projectID,
+	)
+	if err != nil {
+		return nil, err
+	}
+	defer rows.Close()
+	out := []IOMarker{}
+	for rows.Next() {
+		var m IOMarker
+		var frame sql.NullInt64
+		var ex sql.NullString
+		if err := rows.Scan(&m.ID, &m.ProjectID, &frame, &m.Label, &m.X, &m.Y,
+			&ex, &m.CreatedAt, &m.UpdatedAt); err != nil {
+			return nil, err
+		}
+		if frame.Valid {
+			v := frame.Int64
+			m.FrameID = &v
+		}
+		if ex.Valid {
+			m.ExcalidrawID = &ex.String
+		}
+		out = append(out, m)
+	}
+	return out, rows.Err()
+}
+
+// UpdateIOMarker applies a partial update. project_id is locked; frame_id
+// tri-state mirrors DeviceUpdate.FrameID.
+func (s *Store) UpdateIOMarker(projectID, id int64, u IOMarkerUpdate) (*IOMarker, error) {
+	cur, err := s.GetIOMarker(projectID, id)
+	if err != nil {
+		return nil, err
+	}
+	if u.Label != nil {
+		v := strings.TrimSpace(*u.Label)
+		if v == "" {
+			return nil, fmt.Errorf("%w: label cannot be empty", ErrInvalidInput)
+		}
+		cur.Label = v
+	}
+	if u.X != nil {
+		cur.X = *u.X
+	}
+	if u.Y != nil {
+		cur.Y = *u.Y
+	}
+	if u.FrameID.Set {
+		if u.FrameID.ID != nil {
+			if _, err := s.GetFrame(projectID, *u.FrameID.ID); err != nil {
+				if errors.Is(err, ErrNotFound) {
+					return nil, fmt.Errorf("%w: frame_id %d not in project %d", ErrInvalidInput, *u.FrameID.ID, projectID)
+				}
+				return nil, err
+			}
+		}
+		cur.FrameID = u.FrameID.ID
+	}
+	if _, err := s.db.Exec(
+		`UPDATE io_markers
+		 SET frame_id = ?, label = ?, x = ?, y = ?, updated_at = datetime('now')
+		 WHERE id = ? AND project_id = ?`,
+		nullableInt64(cur.FrameID), cur.Label, cur.X, cur.Y, id, projectID,
+	); err != nil {
+		return nil, mapWriteErr(err)
+	}
+	return s.GetIOMarker(projectID, id)
+}
+
+// DeleteIOMarker removes an IO marker from a project.
+func (s *Store) DeleteIOMarker(projectID, id int64) error {
+	if _, err := s.GetIOMarker(projectID, id); err != nil {
+		return err
+	}
+	if _, err := s.db.Exec(
+		`DELETE FROM io_markers WHERE id = ? AND project_id = ?`, id, projectID,
+	); err != nil {
+		return err
+	}
+	return nil
+}
diff --git a/internal/db/io_markers_test.go b/internal/db/io_markers_test.go
new file mode 100644
index 0000000..b1339ec
--- /dev/null
+++ b/internal/db/io_markers_test.go
@@ -0,0 +1,113 @@
+package db
+
+import (
+	"errors"
+	"testing"
+)
+
+func TestCreateIOMarker_DefaultsLabel(t *testing.T) {
+	s := newTestStore(t)
+	p, _ := s.CreateProject("LOFT", "", "")
+	m, err := s.CreateIOMarker(p.ID, IOMarkerCreate{X: 10, Y: 20})
+	if err != nil {
+		t.Fatalf("create: %v", err)
+	}
+	if m.Label != "IO" {
+		t.Errorf("default label = %q, want IO", m.Label)
+	}
+	if m.FrameID != nil {
+		t.Errorf("frame_id = %v, want nil", m.FrameID)
+	}
+}
+
+func TestCreateIOMarker_CustomLabel(t *testing.T) {
+	s := newTestStore(t)
+	p, _ := s.CreateProject("LOFT", "", "")
+	m, err := s.CreateIOMarker(p.ID, IOMarkerCreate{Label: "Wall A", X: 0, Y: 0})
+	if err != nil {
+		t.Fatalf("create: %v", err)
+	}
+	if m.Label != "Wall A" {
+		t.Errorf("label = %q, want Wall A", m.Label)
+	}
+}
+
+func TestCreateIOMarker_CrossProjectFrameRejected(t *testing.T) {
+	s := newTestStore(t)
+	p1, _ := s.CreateProject("LOFT", "", "")
+	p2, _ := s.CreateProject("OFFICE", "", "")
+	f2, _ := s.CreateFrame(p2.ID, FrameCreate{Name: "desk", Width: 100, Height: 50})
+	_, err := s.CreateIOMarker(p1.ID, IOMarkerCreate{FrameID: &f2.ID, X: 0, Y: 0})
+	if !errors.Is(err, ErrInvalidInput) {
+		t.Errorf("cross-project frame_id should ErrInvalidInput; got %v", err)
+	}
+}
+
+func TestGetIOMarker_WrongProjectIsNotFound(t *testing.T) {
+	s := newTestStore(t)
+	p1, _ := s.CreateProject("LOFT", "", "")
+	p2, _ := s.CreateProject("OFFICE", "", "")
+	m, _ := s.CreateIOMarker(p1.ID, IOMarkerCreate{X: 0, Y: 0})
+	if _, err := s.GetIOMarker(p2.ID, m.ID); !errors.Is(err, ErrNotFound) {
+		t.Errorf("cross-project GetIOMarker should be ErrNotFound; got %v", err)
+	}
+}
+
+func TestUpdateIOMarker_FrameIDTriState(t *testing.T) {
+	s := newTestStore(t)
+	p, _ := s.CreateProject("LOFT", "", "")
+	f, _ := s.CreateFrame(p.ID, FrameCreate{Name: "desk", Width: 100, Height: 50})
+	m, _ := s.CreateIOMarker(p.ID, IOMarkerCreate{FrameID: &f.ID, X: 0, Y: 0})
+
+	// Leave alone — passing a different X must not clear frame_id.
+	nx := 99.0
+	u1, _ := s.UpdateIOMarker(p.ID, m.ID, IOMarkerUpdate{X: &nx})
+	if u1.FrameID == nil || *u1.FrameID != f.ID {
+		t.Errorf("frame_id should still be set (Set=false); got %v", u1.FrameID)
+	}
+
+	// Clear.
+	u2, _ := s.UpdateIOMarker(p.ID, m.ID, IOMarkerUpdate{FrameID: FrameRef{Set: true, ID: nil}})
+	if u2.FrameID != nil {
+		t.Errorf("frame_id should be nil after clear; got %v", *u2.FrameID)
+	}
+}
+
+func TestDeleteFrame_SetsIOMarkerFrameIDToNull(t *testing.T) {
+	s := newTestStore(t)
+	p, _ := s.CreateProject("LOFT", "", "")
+	f, _ := s.CreateFrame(p.ID, FrameCreate{Name: "desk", Width: 800, Height: 600})
+	m, _ := s.CreateIOMarker(p.ID, IOMarkerCreate{FrameID: &f.ID, X: 10, Y: 20})
+	if m.FrameID == nil {
+		t.Fatalf("pre-condition: io marker should have frame_id")
+	}
+	if err := s.DeleteFrame(p.ID, f.ID); err != nil {
+		t.Fatalf("delete frame: %v", err)
+	}
+	m2, _ := s.GetIOMarker(p.ID, m.ID)
+	if m2.FrameID != nil {
+		t.Errorf("io marker frame_id post-delete = %v, want nil (SET NULL)", m2.FrameID)
+	}
+}
+
+func TestSnapshot_PopulatesIOMarkers(t *testing.T) {
+	s := newTestStore(t)
+	p, _ := s.CreateProject("LOFT", "", "")
+	_, _ = s.CreateIOMarker(p.ID, IOMarkerCreate{Label: "Wall A", X: 10, Y: 20})
+	_, _ = s.CreateIOMarker(p.ID, IOMarkerCreate{Label: "UPS rear", X: 100, Y: 200})
+	snap, err := s.Snapshot(p.ID)
+	if err != nil {
+		t.Fatalf("snapshot: %v", err)
+	}
+	if len(snap.IOMarkers) != 2 {
+		t.Errorf("io_markers len = %d, want 2", len(snap.IOMarkers))
+	}
+}
+
+func TestDeleteIOMarker_NotFound(t *testing.T) {
+	s := newTestStore(t)
+	p, _ := s.CreateProject("LOFT", "", "")
+	if err := s.DeleteIOMarker(p.ID, 999); !errors.Is(err, ErrNotFound) {
+		t.Errorf("got %v, want ErrNotFound", err)
+	}
+}
diff --git a/internal/db/models.go b/internal/db/models.go
index 228ec4d..51fe3c8 100644
--- a/internal/db/models.go
+++ b/internal/db/models.go
@@ -58,7 +58,7 @@ type Snapshot struct {
 	Devices    []Device    `json:"devices"`
 	Ports      []any       `json:"ports"`
 	Cables     []any       `json:"cables"`
-	IOMarkers  []any       `json:"io_markers"`
+	IOMarkers  []IOMarker  `json:"io_markers"`
 	Bundles    []any       `json:"bundles"`
 	CableTypes []CableType `json:"cable_types"`
 }
diff --git a/internal/db/store.go b/internal/db/store.go
index c6d6056..fb3a5da 100644
--- a/internal/db/store.go
+++ b/internal/db/store.go
@@ -167,13 +167,17 @@ func (s *Store) Snapshot(id int64) (*Snapshot, error) {
 	if err != nil {
 		return nil, err
 	}
+	ios, err := s.ListIOMarkers(id)
+	if err != nil {
+		return nil, err
+	}
 	return &Snapshot{
 		Project:    *p,
 		Frames:     frames,
 		Devices:    devices,
 		Ports:      []any{},
 		Cables:     []any{},
-		IOMarkers:  []any{},
+		IOMarkers:  ios,
 		Bundles:    []any{},
 		CableTypes: types,
 	}, nil