diff --git a/src/presa_mano/geometry.py b/src/presa_mano/geometry.py
index 17dd8e3..b675725 100644
--- a/src/presa_mano/geometry.py
+++ b/src/presa_mano/geometry.py
@@ -109,3 +109,24 @@ def transform_points(T: np.ndarray, pts: np.ndarray) -> np.ndarray:
     R = T[:3, :3]
     t = T[:3, 3]
     return pts @ R.T + t
+
+
+def rpy_deg_to_quaternion_xyzw(roll_deg: float, pitch_deg: float, yaw_deg: float) -> np.ndarray:
+    """Extrinsic XYZ Euler (deg) -> quaternion xyzw. Inverse of `quaternion_xyzw_to_rpy_deg`.
+    Convenzione coerente con `R_to_euler_xyz` (XYZ extrinsic, in radianti)."""
+    cr, sr = np.cos(np.radians(roll_deg) * 0.5), np.sin(np.radians(roll_deg) * 0.5)
+    cp, sp = np.cos(np.radians(pitch_deg) * 0.5), np.sin(np.radians(pitch_deg) * 0.5)
+    cy, sy = np.cos(np.radians(yaw_deg) * 0.5), np.sin(np.radians(yaw_deg) * 0.5)
+    qw = cr * cp * cy + sr * sp * sy
+    qx = sr * cp * cy - cr * sp * sy
+    qy = cr * sp * cy + sr * cp * sy
+    qz = cr * cp * sy - sr * sp * cy
+    return np.array([qx, qy, qz, qw], dtype=np.float64)
+
+
+def quaternion_xyzw_to_rpy_deg(quat_xyzw) -> tuple[float, float, float]:
+    """Quaternion xyzw -> extrinsic XYZ Euler (deg). Convention matches `R_to_euler_xyz`."""
+    R = quat_xyzw_to_R(np.asarray(quat_xyzw, dtype=np.float64))
+    rpy_rad = R_to_euler_xyz(R)
+    rpy_deg = np.degrees(rpy_rad)
+    return float(rpy_deg[0]), float(rpy_deg[1]), float(rpy_deg[2])
diff --git a/tests/test_geometry_rpy.py b/tests/test_geometry_rpy.py
new file mode 100644
index 0000000..6cb7887
--- /dev/null
+++ b/tests/test_geometry_rpy.py
@@ -0,0 +1,40 @@
+"""Roundtrip RPY (deg) ↔ quaternion xyzw."""
+from __future__ import annotations
+
+import numpy as np
+import pytest
+
+from presa_mano.geometry import (
+    quaternion_xyzw_to_rpy_deg,
+    rpy_deg_to_quaternion_xyzw,
+)
+
+
+def test_zero_rpy_is_identity_quat():
+    q = rpy_deg_to_quaternion_xyzw(0.0, 0.0, 0.0)
+    assert np.allclose(q, [0.0, 0.0, 0.0, 1.0], atol=1e-9)
+
+
+def test_quat_identity_is_zero_rpy():
+    rpy = quaternion_xyzw_to_rpy_deg([0.0, 0.0, 0.0, 1.0])
+    assert np.allclose(rpy, [0.0, 0.0, 0.0], atol=1e-6)
+
+
+@pytest.mark.parametrize("roll,pitch,yaw", [
+    (10.0, 20.0, 30.0),
+    (-45.0, 0.0, 90.0),
+    (0.0, 60.0, -30.0),
+    (15.0, -25.0, 80.0),
+])
+def test_roundtrip_rpy_quat_rpy(roll, pitch, yaw):
+    q = rpy_deg_to_quaternion_xyzw(roll, pitch, yaw)
+    r2, p2, y2 = quaternion_xyzw_to_rpy_deg(q)
+    assert abs(r2 - roll) < 1e-4
+    assert abs(p2 - pitch) < 1e-4
+    assert abs(y2 - yaw) < 1e-4
+
+
+def test_quat_unit_norm():
+    q = rpy_deg_to_quaternion_xyzw(33.0, -17.0, 91.5)
+    n = float(np.linalg.norm(q))
+    assert abs(n - 1.0) < 1e-9