Abstract Background Evidence supporting combination treatment with a beta-lactam plus an aminoglycoside (C-BA) for endocarditis caused by viridans and gallolyticus group streptococci (VGS-GGS) with intermediate susceptibility to penicillin (PENI-I) is lacking. We assessed the clinical characteristics and outcomes of PEN-I VGS-GGS endocarditis and compared the effectiveness and safety of C-BA with third-generation cephalosporin monotherapy. Methods Retrospective analysis of prospectively collected data of a cohort of definite endocarditis caused by penicillin-susceptible and PENI-I VGS-GGS (penicillin minimum inhibitory concentration ranging from 0.25 to 2 mg/L) between 2008 and 2018 in 40 Spanish hospitals. We compared cases treated with monotherapy or with C-BA and performed multivariable analyses of risk factors for in-hospital and 1-year mortality. Results A total of 914 consecutive cases of definite endocarditis caused by VGS-GGS with complete or intermediate susceptibility to penicillin were included. A total of 688 (75.3%) were susceptible to penicillin and 226 (24.7%) were PENI-I. Monotherapy was used in 415 (45.4%) cases (cephalosporin in 331 cases) and 499 (54.6%) cases received C-BA. In-hospital mortality was 11.9%, and 190 (20.9%) patients developed acute kidney injury. Heart failure (odds ratio OR: 6.06; 95% confidence interval CI: 1.37–26.87; P = .018), central nervous system emboli (OR: 9.83; 95% CI: 2.17–44.49; P = .003) and intracardiac abscess (OR: 13.47; 95% CI: 2.24–81.08; P = .004) were independently associated with in-hospital mortality among PEN-I VGS-GGS cases, while monotherapy was not (OR: 1.01; 95% CI: .26–3.96; P = .982). Conclusions Our findings support the use of cephalosporin monotherapy in PEN-I VGS-GGS endocarditis in order to avoid nephrotoxicity without adversely affecting patient outcomes. In a retrospective analysis of endocarditis due to viridans and gallolyticus groups streptococci, cephalosporin monotherapy was not associated with higher in-hospital mortality when the isolate showed intermediate susceptibility to penicillin.