A lens structure, a light source device, and a light source module are provided. The light source device includes a light emitting device and a lens structure. The light emitting device is capable of emitting a light beam. The lens structure includes a first surface, a second surface opposite to the first surface and four total internal reflection surfaces connected to the second surface. Some of the total internal reflection surfaces connect to the first surface. The first surface has a recess. The light emitting device is disposed at the recess. The second surface is a free-form surface. The light beam is capable of entering the lens structure through the first surface, and leaving the lens structure through the second surface.