Abstract Background Trust is described as the glue of healthy societies. Although Norway is among the countries with the highest levels of trust in public institutions, the increasing socioeconomic differences observed may threaten levels of institutional trust. Therefore, we aimed to investigate such differences in the general population between 2011 and 2019. Methods A repeated cross-sectional study was conducted using data on the general population in Østfold county from 2011 (N = 16673) and 2019 (N = 15935). Differences in trust toward five public institutions (healthcare system, school system, labour and welfare administration (NAV), police, and municipal council) by educational level were investigated with Chi-square tests. Logbinomial regression analyses were used to examine potential changes in high trust over time. Gender and age were included in the analyses as control variables. Results A relatively high proportion of the general population reported high levels of trust to the health care system (2011: 71%; 2019:80%), police (2011: 73%; 2019:77%), and the school system (2011: 64%; 2019:66%), whereas a lower proportion reported high levels of trust to NAV (2011: 37%; 2019:41%) and the municipal council (2011: 45%; 2019:36%). Both in 2011 and 2019 a clear social gradient in institutional trust was found for all institutions (p < 0.05). There was a significant increase in the level of trust in the health care system (low education:RR=1,10; middle:RR=1,12; high:RR=1.06) and a significant decrease in trust in the community council (low:RR=0,77; middle:RR=0,80; high:RR=0,83) in all educational groups from 2011 to 2019. The level of trust in NAV increased significantly in the highest education groups, while a non-significant decrease was shown in the low educational group. Conclusions The persisting social gradient in institutional trust is worrying as trust is an important factor in ensuring inclusion, social capital, democracy, and health and wellbeing. Key messages • Institutional trust is an important factor in ensuring inclusion, social capital, democracy, and health and wellbeing. • The lower levels of institutional trust found in low educational groups could be of particular concern as these groups might be more dependent on the different parts of the welfare system.