Military officials say they will register weight before professional exams, to link promotion and fitness.
Defence Minister Ruben Saavedra has said soldiers "must be prepared both on an intellectual and physical level".
And the police have forbidden overweight members to wear uniforms until they slim down.
The move may already have delivered results, if unexpected ones.
In the town of Oruro south of La Paz, two policemen patrolling in civilian clothing succeeded in catching some thieves in a street market, apparently because they weren't in uniform.
On 2 December, President Evo Morales suggested that soldiers and officers who neglected their fitness should not be promoted, insisting that staying in good shape was a "responsibility" of the security forces.
Soldiers and police have complained of a lack of sports facilities, so the government has been supplying equipment to barracks and police stations.
Bolivia's security forces have begun registering overweight members after President Evo Morales complained that too many of them were out of shape.