Background: Only 17% of Latinas meet national physical activity (PA) guidelines for both moderate-to-vigorous aerobic and muscle-strengthening PA. Additional health benefits are derived from the combination of aerobic and muscle-strengthening PA (vs. aerobic alone), yet there is paucity in research on muscle-strengthening activity in Latinas. The aim of this study was to examine changes in muscle-strengthening activity from baseline to 6 and 12 months in "Seamos Saludables," a 12-month PA randomized controlled trial for Latinas. Methods: A secondary data analysis was conducted among 131 Latinas ages 18-65 years, who were randomized to either a PA Intervention or a Wellness Control. Self-reported muscle-strengthening exercise was measured at baseline, 6 months, and 12 months via adapted muscle-strengthening questions from the Behavioral Risk Factor Surveillance System. Results: There was a 16-minute/week difference in median minute/week of muscle-strengthening activity between Intervention and Wellness at 6 months (SE = 7.91, p = 0.04) and 45-minute/week difference at 12 months (SE = 25.80, p = 0.06) adjusting for baseline. Significantly more PA Intervention participants met muscle-strengthening guidelines of 2 or more days/week at 6 months versus Wellness Control participants (odds ratio [OR] = 4.29, 95% confidence interval [CI] = [1.03, 17.84]). Conclusion: Results from the current study showed that Latinas engaged in muscle-strengthening activity in an intervention that emphasized primarily aerobic PA outcomes, suggesting they may be interested in engaging in muscle-strengthening activities. Future interventions targeting both aerobic and muscle-strengthening activity could achieve greater health improvements and help more Latinas reach the full national PA guidelines.