Objectives The incidence of sexually transmitted infections (STIs) and HIV infection remains high in gay, bisexual, and other men who have sex with men (MSM) in the UK, and sexualized drug use (“chemsex”) and injecting drug use (“slamsex”) may play a part in this. We aimed to characterize HIV‐positive MSM engaging in chemsex/slamsex and to assess the associations with self‐reported STI diagnoses and sexual behaviours. Methods Data from a 2014 survey of people attending HIV clinics in England and Wales were linked to clinical data from national HIV surveillance records and weighted to be nationally representative. Multivariable logistic regression assessed the associations of chemsex and slamsex with self‐reported unprotected anal intercourse (UAI), serodiscordant UAI (sdUAI) (i.e. UAI with an HIV‐negative or unknown HIV status partner), sdUAI with a detectable viral load (>50 HIV‐1 RNA copies/mL), hepatitis C, and bacterial STIs. Results In the previous year, 29.5% of 392 sexually active participants engaged in chemsex, and 10.1% in slamsex. Chemsex was significantly associated with increased odds of UAI adjusted odds ratio (AOR) 5.73; P < 0.001, sdUAI (AOR 2.34; P < 0.05), sdUAI with a detectable viral load (AOR 3.86; P < 0.01), hepatitis C (AOR 6.58; P < 0.01), and bacterial STI diagnosis (AOR 2.65; P < 0.01). Slamsex was associated with increased odds of UAI (AOR 6.11; P < 0.05), hepatitis C (AOR 9.39; P < 0.001), and bacterial STI diagnosis (AOR 6.11; P < 0.001). Conclusions Three in ten sexually active HIV‐positive MSM engaged in chemsex in the past year, which was positively associated with self‐reported depression/anxiety, smoking, nonsexual drug use, risky sexual behaviours, STIs, and hepatitis C. Chemsex may therefore play a role in the ongoing HIV and STI epidemics in the UK.