Sarcopenia (loss of muscle mass/strength) burdens many older adults – hospitalized older adults being particularly vulnerable. Treating the condition, protein-supplementation (PrS) and resistance-training (RT) may act synergistically. Therefore, this block-randomised, double-blind, multicentre intervention study, recruiting geriatric patients >70 years from three Medical Departments, investigated the effect of PrS combined with RT during hospitalization and 12 weeks after discharge. Participants were randomly allocated (1:1) to receive PrS (totally 27.5 g whey protein/day, ≈2000 kJ/day) or iso-energetic placebo-products (<1.5 g protein/day), divided into two servings/day to supplement habitual diet. Both groups were engaged in a standardized, progressive low intensity/volume RT-program for the lower-extremities (hospital: supervised daily/after discharge: self-training 4x/week). From April 2016 to September 2017, 2351 patients were screened, 462 were eligible, and 165 included. 14 were excluded and 10 dropped-out, leaving 141 participants in the ITT-analysis. The average total protein intake during hospitalization/after discharge was 1.0 (0.8-1.3)/1.1 (0.9–1.3) g/kg/d (protein-group) and 0.6 (0.5–0.8)/ 0.9 (0.6–1.0) g/kg/d (placebo-group). Both groups improved significantly for the primary and secondary endpoints of muscle mass/strength, functional measurements, and quality-of-life, but no additional effect of PrS was seen for the primary endpoint (30-s Chair-stand-test, repetitions, median (Q1,Q3) changes from baseline: (standard-test: 0 (0,5) (protein-group) vs. 2 (0,6) (placebo-group) & modified-test: 2 (0,5) (protein-group) vs. 2 (-1,5) (placebo-group)) or any secondary endpoints (Mann-Whitney-U tests, P>0.05). In conclusion, PrS increasing total protein intake by 0.4 and 0.2 g/kg/d during hospitalization and after discharge, respectively, does not seem to increase the adaptive response to low intensity/volume RT in geriatric medical patients.