This paper studies experimentally and analytically the influence of the viscoelastic cardiac muscle on the energy harvesting from heartbeats for powering the leadless pacemakers by using a piezoelectric beam. An appropriate representative gel-type viscoelastic material that mimics the heart tissue is used in the tests. The piezoelectric beam coupled with a gel-type material is analytically modeled, and experimentally tested. By considering a combination of the translational standard linear solid model and a rotational spring component for the gel-type material, the analytical model of the coupled system is developed utilizing the generalized Hamilton’s principle. The system is attached on top of a shaker and excited harmonically, and the time history of output voltage and accelerations are measured. The analytical model is verified by experimental results for the tip displacement, voltage, generated power, phase-portraits histories, and voltage FRF with harmonic base excitations. Transmissibility analysis by the analytical model shows that for excitation frequencies beyond a specific frequency, the viscoelastic material can magnify the amplitude of the excitation and incredibly improve the power generation. Experimental results demonstrate that by coupling the gel into the harvester, oscillations of the tip are increased into high energy orbits and large tip deflections around the resonance frequency. The significantly widened frequency bandwidth and the increased power output at specific input frequencies are the other results of considering the viscoelastic characteristics of the heart wall in the dynamic investigations. By simulating the response of the energy harvesting system to the heartbeat impulsive rhythm, when the energy harvesting system is attached to the viscoelastic material, the output power is increased from 18 to 55 µW. The obtained results reveal that influence of the viscoelastic properties of the heart muscle is crucial in the accurate design of the energy harvesting system for the self-powered medical leadless pacemaker.