Background Since the 1994 International Conference on Population and Development (ICPD) in Cairo, youth-friendly services have been recognized as an appropriate and effective strategy to address the sexual and reproductive health needs of adolescents. Nearly two decades have passed since Ethiopia has introduced this program in the national health system. Yet, young people in Ethiopia are at a greater risk of a broad range of sexual and reproductive health problems such as unwanted pregnancies, unsafe abortion, and sexually transmitted infections. Individual attributes, health services related, and socio-cultural norms, in general, are identified as factors for adolescents’ sexual behavior. Little is known whether youth-friendly services address these factors, and promoting the sexual and reproductive health of unmarried adolescents in west Gojjam zone, northwest Ethiopia. The finding of this study may have relevance for practitioners and policymakers to improve the sexual and reproductive health of unmarried adolescents in west Gojjam Zone. Objective The purpose of the study was to investigate the role of youth-friendly service program in promoting sexual and reproductive health of unmarried adolescents in West Gojjam Zone, Amhara Region, Ethiopia. Methods The study employed a cross-sectional quantitative study design with sequential mixed methods research approach in the vicinity of health facilities. First, the health facilities were grouped into two based on the presence/absence of youth-friendly service programs in the facilities. Using the Donabedian Model, the quality of youth-friendly services was assessed in 18 out of 54 health centers. Simulated clients, who were trained on different sexual and reproductive health-related scenarios, participated in the study. Data were collected using standard checklists and summarized using descriptive statistics. Then, two phases of the study were conducted sequentially. The first phase was qualitative, aimed at exploring the socio-cultural norms of adolescents' sexuality. The study was conducted in the two settings- youth-friendly services program and non-program areas. Purposive sampling was used to identify study participants. Data were collected using semi-structured in-depth interviews, key informants, and focus group discussion guides. A total of eighteen key informant interviews, twelve focus group discussions, and four in-depth interviews were conducted. Informants comprised of unmarred adolescents, parents, religious leaders, community elders, health professionals, teachers, and unmarred adolescents who experienced sexual and reproductive health problems. In the second phase, community surveys were done in the two settings to assess the sexual behavior of unmarried adolescents and parent-adolescent communication on sexual and reproductive health issues. A total of 2,250 participants (1,125 unmarried adolescents and 1,125 parents) were randomly selected and participated in the study. Data were collected both from unmarried adolescents and their parents using the interviewer-administered technique. The quantitative data were entered and analyzed using SPSS windows version 21. Chi-square test and t-test were used to compare variables between the program and non-program areas. Binary logistic regression analysis was used to identify significant variables for parent-adolescent communication using p < 0.05 as a cutoff, and hierarchical regression analysis was used for the sexual behavior of unmarried adolescents. The qualitative data were analyzed thematically. The study was conducted between May/2018 to March 2019. Results The study revealed that quality of youth-friendly services ranged from low to medium, with adolescent-related elements performing poorly. None of the health facilities were providing good quality of services. Relatively, a promising result was found in the input quality that measured the availability of trained providers, drugs, and supplies. The presence of community-based health insurance and an age-driven comprehensive youth-friendly service delivery approach were identified as challenges to deliver quality services. Regarding sexual behavior of unmarried adolescents, of 1,125 respondents, 305(27.1%) had risky sexual behavior and was comparable between the youth-friendly services program and non-program areas (25.0% Vs 29.1%, p=0.12). However, the proportion of adolescents who had good knowledge of sexual and reproductive health and sexual and reproductive health service utilization were significantly higher in the youth-friendly services intervention area. Including interpersonal-related variables in the hierarchical regression model significantly improved the explanation of risky sexual behavior over and above individual attributes and youth-friendly services intervention-related variables. A point increase in parent-adolescent communication score reduced risky sexual behavior by 20 % (AOR=0.80, 95%CI: 0.75, 0.85). Being female, increasing in age, knowing family planning and HIV, being out of school, and watching pornography were associated with higher odds of engaging in risky sexual behavior. Regarding parent-adolescent communication, 352(31.3%) of parents had good communication on sexual and reproductive health issues with a statistically significant difference between intervention 190 (34.9%) and non-intervention 162(27.9%) areas, (χ2 (1) = 6.28, p=0.01). The odds of parent-adolescent communication were about 45% higher among parents residing in the intervention area than the non-intervention (AOR=1.45; 95%CI: 1.07-1.89, p=0.01). A point increase in parent’s attitude score to discuss SRH issues was associated with higher odds of engaging in communication (AOR=1.12; 95%CI: 1.07-1.15, p<0.001). The qualitative findings indicated that the socio-cultural norm in the two settings is intolerant to premarital sex, sexual and reproductive health service utilization (eg., contraceptive use) by unmarred adolescents; and discourages sexual and reproductive health communication with unmarred adolescents. Informants shared that premarital sex and sexual and reproductive health service use were not accepted by the community and considered a sin. Moreover, the informants believed that having communication on sexual and reproductive health issues with unmarred adolescents encourages them to initiate sex, therefore, should not be practiced. In sum, the communications were limited to abstinence, event-related, unidirectional, and characterized by warning messages. Conclusion A youth-friendly service program implemented in west Gojjam Zone has a promising effect on precursors of healthy sexual behaviors including parent-adolescent communication. However, the socio-cultural norms influencing adolescent sexual health and unmarried adolescent sexual behavior were more or less remaining comparable between settings. Therefore, this study would suggest interventions that should give due emphasis to the socio-cultural norms surrounding adolescent’s sexual and reproductive health. Keywords: Adolescent, sexual behavior, socio-cultural norms, youth-friendly services, parent-adolescent communication