(1) Background: There is a dearth of data on the levels and determinants of testing for drug-related infectious diseases among people who use drugs (PWUD). We assessed the proportions and determinants of testing for drug-related infectious diseases to inform ongoing interventions for PWUD. (2) Methods: A cross-sectional study involving 599 PWUD was conducted in Dar es Salaam and Tanga between January and February 2019. Data were collected through a researcher-administered questionnaire using handheld tablets. Logistic regression models were used to identify independent testing determinants for drug-related infectious diseases. (3) Results: A majority (98.0%) of participants were males, with a mean age of 36.8 (SD = 7.8) years. 75.0%, 40.6%, 38.6%, and 8.2% reported having ever tested for HIV, tuberculosis (TB), sexually transmitted infections (STIs), and viral hepatitis, respectively. The likelihood of HIV testing was higher among those living with someone (AOR = 2.18, 95% CI: 1.09-4.68) compared with those who were homeless and perceived treatment was appropriate (AOR = 2.18, 95% CI: 1.05-4.46), but was lower among those who experienced mild to moderate (AOR = 0.44, 95% CI: 0.21-0.95) and severe internalized stigma (AOR = 0.44, 95% CI: 0.22-0.94) compared with those reporting no internalized stigma, and among those who experienced financial difficulties resulting from spending on health care services (AOR = 0.60, 95% CI: 0.40-0.89). Perception of treatment appropriateness (AOR = 2.29, 96% CI: 1.10-5.06) and severe enacted stigma (AOR = 1.90, 95% CI: 1.06-3.42) were associated with increased odds of TB testing. The odds of STIs testing increased among those who were married (AOR = 2.31, 95% CI: 1.45-3.72) compared with those who were single and those who had experienced mild (AOR = 2.39, 95% CI: 1.28-4.53) or severe (AOR = 6.20, 95% CI: 1.99-23.83) sexual violence, compared with those who had not experienced sexual violence. However, the odds decreased among those who had been remanded in the past month (AOR = 0.64, 95% CI: 0.43-0.95) compared with those who were not remanded and among those who had financial difficulties resulting from spending on health care services (AOR = 0.66, 95% CI: 0.47-0.94). The likelihood of testing for viral hepatitis testing increased among those who had heard about the comprehensive HIV intervention package (CHIP) (AOR = 2.59, 95% CI: 1.40-4.94); however, it decreased among those who had financial difficulties resulting from spending on health care services (AOR = 0.48, 95% CI: 0.24-0.92). (4) Conclusions: Except for HIV, PWUD had undergone limited testing for drug-related infectious diseases. The study findings highlight some factors influencing testing for the selected infectious diseases investigated, which should be targeted for tailored interventions to improve diagnosis and treatment.