The antimicrobial evaluation of twelve natural and hemisynthetic isopimarane diterpenes are reported. The compounds were evaluated against a panel of Gram-positive bacteria, including two methicillin-resistant Staphylococcus aureus (MRSA) strains and one vancomycin-resistant Enterococcus (VRE) strain. Only natural compounds 7,15-isopimaradien-19-ol (1) and 19-acetoxy-7,15-isopimaradien-3β-ol (6) showed promising results. Isopimarane (1) was the most active, showing MIC values between 6.76 µM against S. aureus (ATCC 43866) and 216.62 µM against E. faecalis (FFHB 427483) and E. flavescens (ATCC 49996). Compound (6) showed moderated activity against all tested microorganisms (MIC between value 22.54 and 45.07 µM). These compounds were found to be active against the methicillin-sensitive strains of S. aureus (CIP 106760 and FFHB 29593), showing MIC values of 13.55 (1) and 22.54 (6) µM. Both compounds were also active against vancomycin-resistant E. faecalis (ATCC 51299) (MIC values of 54.14 and 45.07 µM, respectively). In addition, the cytotoxicity of nine compounds 7,15-isopimaradien-3β,19-diol (2); mixture: 15-isopimarene-8β-isobutyryloxy-19-ol and 15-isopimarene-8β-butyryloxy-19-ol (3); 3β-acetoxy-7,15-isopimaradiene-19-ol (5); 19-acetoxy-7,15-isopimaradiene-3β-ol (6); 3β,19-diacetoxy-7,15-isopimaradiene (8); 15-isopimarene-8β,19-diol (9); 19-O-β-d-glucopyranoside-7,15-isopimaradiene (10); lagascatriol-16-O-β-d-glucopyranoside (11) and lagascatriol-16-O-α-d-mannopyranoside (12) was evaluated in the human breast cancer cell line MDA-MB-231. Isopimarane (2) was the only compound showing some cytotoxicity. The IC50 value of compound (2) was 15 µM, suggesting a mild antiproliferative activity against these breast cancer cells.