This descriptive study aimed to explain the reproductive health of urban slum adolescents in the Khon Kaen Municipality area of Khon Kaen, Thailand. A self-reported questionnaire that took about 20 minutes to complete was used for data collection. Multi-stage simple random sampling was adopted in the selection of five target communities to recruit 277 male and female adolescents aged 10–19 years in accordance with the proportion of male and female adolescents in the area. Frequencies, percentages, standard deviations, and means were used for the data analysis. The female and male participants had an average age of 14.62 ± 2.66 years and 14.58 ± 2.84 years, respectively. The average menarcheal age was 12.96 ± 1.58 years, while the age at which the first wet dream was experienced for boys was 14.12 ± 1.44 years. Most of the participants were in elementary school, while 5.7% of female and 2.4% of male adolescents did not attend school. Most girls and boys knew about contraceptive pills and condoms, but not other birth control methods. Contraceptive pills were used by 26.4% of female adolescents and condoms were used by 39.8% of male adolescents. However, it was reported that less than 10% of males and females regularly used condoms. The lowest age of the 36.5% of girls who had had a sexual experience was 10 years, whereas the lowest age of the 40.7% of sexually experienced boys was 11 years. The average age of the group of females who had started to have sexual intercourse was 14.81 ± 1.71 years, and the average age of the group of males who had had sexual intercourse was 15.23 ± 1.32 years. Most of the sample had had intercourse with their girlfriends or boyfriends. It was found that 1.3% of the girls and 1.8% of the boys were prostitutes and that 10.9% of the boys had visited brothels. Twenty-two percent of the girls admitted that they had masturbated, while 41.0% of the males did. About 17.8% of the female adolescents had been pregnant; 50% of those pregnancies ended in abortion and 50% of the females had been pregnant more than once.