Controlling the spread of HIV requires essential knowledge. This study explores how sociodemographic factors relate to HIV transmission knowledge among male drug users in Malaysia. This study utilised cross-sectional data from the Integrated Biological and Behavioral Surveillance Survey (IBBS) 2017 (n = 1,413). An ordered probit regression was used to evaluate parameters influencing the probabilities of having low-, middle- and high-level knowledge of HIV transmission. Almost half of drug users had low- and middle-level knowledge of HIV transmission. The likelihood of possessing low- and middle-level knowledge about HIV transmission were 2.1%–8.3% lower among drug users aged 30–49 than those aged ≥50. Drug users with secondary- and tertiary-level education were 5.8%–14.9% more likely to possess high-level knowledge about HIV transmission compared to individuals with primary-level education. The odds of possessing high-level knowledge of HIV transmission was 6.3% higher among divorced/widowed drug users, while it was 23%–32.9% lower among non-Malays when compared with others. In conclusion, drug users are more likely to have poor knowledge about HIV transmission if they are older, less educated, non-Malays and single. Future HIV-control initiatives should give particular consideration to the populations with these sociodemographic traits.