Considering the general population's lack of knowledge and, more importantly, the hidden proportion of people living with HIV (PLWHIV), we aimed to study people's knowledge of HIV transmission modes and investigate the history of rapid test usage in the south of Tehran, Iran. We conducted a telephone survey study on 1311 individuals aged 18 years and older. We asked the participants if they had undergone HIV testing at private or public health centers in the past year. Using five items, we assessed HIV transmission knowledge. We used multivariable logistic regression to analyze the relationship between various factors associated with HIV transmission knowledge. Overall, 40.9% of the general population demonstrated a high level of understanding about HIV transmission, with a 95% CI [37.85,44.12]. The result of multivariable logistic analysis revealed that the following factors were associated with better knowledge: higher education (OR = 7.83
[4.72,13.01] for academic and OR = 4.52
[2.78,7.35] for diploma), housekeeping (OR = 1.95
[1.47,2.59]), higher socioeconomic status (OR = 4.41
[3.01,6.15] for middle and OR = 5.46
[2.91,10.15] for middle upward), recent visits to the health center (OR = 1.29
CI [0.98,1.65]), and tobacco use (OR = 1.48
[1.07,2.06]). The knowledge of HIV transmission was negatively associated with age (OR = 0.97
[0.95,0.98]). In addition, 9.8% [7.31,12.87] of participants-4.7% in private clinics and 5.1% in public centers-had received an HIV test within the 12 months prior to the study. Our study emphasizes the importance of improving public knowledge about HIV transmission and expanding testing initiatives. Findings advocate for enhanced educational programs, increased accessibility to testing facilities, and broader public health campaigns. These measures are essential for fostering a well-informed and proactive community in efforts to address HIV/AIDS.