Full text article. Also available at: http://dx.doi.org/10.1136/bmjopen-2020-040450This study was undertaken to assess the trend and contributors of socioeconomic inequalities in antenatal care (ANC) utilisation among women of reproductive age in Tanzania from 2004 to 2016. This study analysed nationally representative data for women of reproductive age obtained from the 2004–2016 Tanzania Demographic Health Surveys. The outcome variables analysed in this study are: (1) attendance of ANC and (2) accessing adequate antenatal care. The concentration curve and the concentration index were used to measure socioeconomic inequality in attending and accessing adequate ANC. The concentration index was decomposed to identify the factors explaining the observed socioeconomic inequality of these two outcomes. The concentration index for attending at least four ANC visits increased from 0.169 in 2004 to 0.243 in 2016 (p<0.01). Similarly, for accessing adequate care, the index increased from 0.147 in 2004 to 0.355 in 2016 (p<0.01). This indicates the significant increase in socioeconomic inequalities (favouring wealthier women) for these two outcomes over time. Furthermore, the results show that wealth status was the largest contributor to inequality in both attending at least four visits (71%, 50% and 70%) and accessing adequate ANC (50%, 42% and 51%) in 2004, 2010 and 2016, respectively, in favour of wealthier women (p<0.05). The other contributors to socioeconomic inequalities in ANC utilisation were maternal education and type of residence. Over the 12 years of surveys, there was no reduction in socioeconomic inequalities in ANC utilization in Tanzania. Therefore, the efforts of achieving universal health coverage should focus on reducing wealth-related inequality and improving women’s education from poor households.