Mozambique is one of the countries in sub-Saharan Africa most affected by the HIV epidemic. Multiple and concurrent sexual partnerships (MSP/CP) have been recognized as one of the key drivers in the rapid spread of HIV in the region. Though HIV prevention programs have been successful in increasing condom use and HIV testing, reducing the practice of MSP/CP has been more difficult. Grounding their interventions in social and behavior change theory, four organizations in Mozambique joined efforts to implement a year-long, multimedia national campaign for HIV prevention with emphasis on the reduction of MSP/CP. Evaluating its impact and identifying the factors that hinder or contribute to its success are critical to building effective programs in the future. With data from a 2011 population-based survey of 1427 sexually active women and men, multivariate causal attribution (MCA) analysis was used to estimate the impact of the campaign in the four regions of Mozambique with the highest levels of HIV prevalence. The analysis tested the psychosocial pathways through which the campaign was expected to affect MSP. The results indicate that exposure (recall) was high; 81.2% of the respondents could recall one or more of the communication campaign components. The campaign had a significant indirect impact on MSP through its negative effect on attitudes that favor MSP, and its positive effect on knowledge and discussion of MSP risk with sex partner. This study demonstrates the value of identifying appropriate psychosocial factors and using them to design the campaign communication strategy, and evaluate the causal pathways by which it has an impact. The campaign was successful in changing MSP behavior by working through two psychosocial variables.