According to one embodiment, a strain sensor includes: a base; a strain sensing element; a magnetic field sensing element; and a processing unit. The strain sensing element includes a first magnetic layer having a first magnetization; a second magnetic layer having a second magnetization; and a first intermediate layer. In the strain sensing element an angle between a direction of the first magnetization and a direction of the second magnetization changes in accordance with a strain. The magnetic field sensing element includes a third magnetic layer having a third magnetization; a fourth magnetic layer having a fourth magnetization; and a second intermediate layer. In the magnetic field sensing an angle between a direction of the third magnetization and a direction of the fourth magnetization changes in accordance with a magnetic field.