This mixed-method study aimed to describe HIV stigmatizing attitudes, identify factors associated with stigmatizing attitudes, and explore the broader context of HIV stigma among men accompanying their pregnant female partners to antenatal care in Tanzania. The study recruited 480 men who were attending a first antenatal care appointment with their pregnant female partners. Participants completed a structured survey; a subset of 16 men completed in-depth interviews. The majority of participants endorsed at least one of the stigmatizing attitudes; the most common attitude endorsed was the perception that HIV is a punishment for bad behaviour. In a multivariable logistic analysis, men were more likely to endorse stigmatizing attitudes if they were younger, less educated, Muslim, did not know anyone with HIV, or reported less social support. In the qualitative interviews, men discussed how HIV was antithetical to masculine identities related to respect, strength, independence, and emotional control. Future studies should develop and test interventions to address HIV stigmatizing attitudes among men, taking advantage of settings of routine HIV testing. These programs should be tailored to reflect masculine ideals that perpetuate stigma.