Bone marrow plays an important role on the mechanical properties of trabecular bone. Its effect on the mechanical properties of porcine trabecular bone is studied in this paper. Uniaxial compression at a low strain rate (0.01 s−1 to 20% strain) and stress relaxation tests (600 s at 85, 70 and 55% of the max. load) were done on 90 different femur samples. Half the samples were treated to extract the bone marrow. The average pore size of the trabecular network was 0.280 ± 0.056 mm. Higher values of elastic modulus (37%), 0.2% yield stress (48%), maximum stress (39%), strain at maximum stress (54%), and toughness (300%), were found for the samples which had the bone marrow extracted and were saturated with a saline solution. A linear relation between the applied load and the relaxation stress of ${\sigma }_{rel}=0.76{\sigma }_{o}$ was found, which means that the trabecular bone behaves as a linear viscoelastic material. A mathematical approximation of the relaxation response was done using a Kohlrausch-Williams-Watts model for viscoelastic materials. Results show that it is essential to consider the viscoelastic behavior that the marrow has on the mechanical properties of the trabecular bone. The effect that the bone marrow has on the stress relaxation was found to be negligible at low strain rates and in the elastic stage of deformation.