•We performed a prospective study in PWUDs from one of the largest shantytowns in Europe.•The prevalence of active HCV infection was high (23.1%), particularly in IDUs (40.2%).•The HCV care cascade was poor; only around 50% of HCV viremic patients began HCV therapy.•The prevalence of HBsAg was 4.3%, and only two individuals had anti-HDV antibodies. The burden of hepatitis C virus (HCV) infection among people who use drugs (PWUDs) is considerable. We aimed to screen for HCV infection using the fingerstick dried blood spot (DBS) test and to describe the cascade of hepatitis C care among PWUDs in Madrid, Spain. We also evaluated the prevalence of hepatitis B virus (HBV) and hepatitis D virus (HDV) in this population. We carried out a prospective study and collected samples and epidemiological data using a mobile unit. Viral infections were tested by immunoassay and RT-PCR assay. PWUDs with a positive result were contacted and referred to a specialized health center to confirm and treat the HCV infection. We studied 529 PWUD samples; 49.7% were from persons who had previously used injection drugs (IDUs). Of these, 152 (28.7%) were positive for HCV antibodies, 122 (23.1%) for HCV RNA, 23 (4.3%) for HBsAg, and two (0.4%) for HDV antibodies (8.7% of those with hepatitis B). People who inject drugs (PWID) more frequently had positive HCV antibody titers (52% vs. 7.3%; p<0.001) and a positive HCV RNA test result (40.2% vs. 7.3%; p<0.001) than non-PWID. The time from sample collection to test results was 19 days. The next 104 individuals (85.2%) with active HCV infection were contacted to report their HCV test results. Of these, 63 (51.6%) had an appointment, 62 (50.8%) were evaluated in the hospital, and 56 (45.9%) started HCV therapy. HCV screening using fingerstick DBS was an excellent tool for determining HCV prevalence and other chronic hepatitis viruses (HBV and HDV) in PWUDs. However, linkage to care was limited, mainly with respect to the initiation of HCV therapy.