Inter-pregnancy interval is an important determinant of both maternal and child health. Suboptimal inter-pregnancy interval has been associated with adverse maternal outcomes -including postpartum hemorrhage and hypertensive disorders, direct causes of maternal mortality. Both overall maternal mortality and the contribution of postpartum hemorrhage on maternal mortality have increased in Tanzania. If we are to achieve sustainable development goal (SDG) number 3.1 to reduce the global maternal mortality ration to less than 70 per 100,000 live births by 2030, it is highly important that such contributors are dealt with. This study aimed to determine the distribution and trends of inter-pregnancy interval and to deduce its association with adverse maternal outcomes among women who delivered at Kilimanjaro Christian Medical Centre (2000-2015).A retrospective cohort study was designed using Kilimanjaro Christian Medical Centre medical birth registry data for women who delivered from 2000 to 2015. Women with at least two births recorded in the registry were included. A total of 7,995 births from 6,612 mothers were analyzed. Anemia during pregnancy, post-partum hemorrhage and pre-eclampsia were adverse maternal outcomes of interest. Data analysis was performed using multivariable logistic regression models allowing for robust standard errors. Crude and adjusted odds ratio with their respective 95% confidence intervals were estimated. More than half (51.7%) of non-first births were born within sub-optimal IPI. The median IPI was 34 months (IQR: 33.5 months). The median IPI increased from 11 months in 2002 to 35 months in 2006 and plateaued until 2014, but it rose to 41.6 months in 2015. Median IPI was shorter in young women <20 years and in birth order seven and above (16 vs. 27 months, respectively). Short IPI was associated with lower risk of pre-eclampsia [aOR: 0.71, 95%CI: 0.52, 0.97] while long IPI was associated with lower risk of post-partum hemorrhage [aOR: 0.70, 95%CI: 0.52, 0.94]. This study found an association between long and short IPI with adverse maternal outcomes. Even though these results should be interpreted with caution based on the fact that the data was sampled from a referral hospital and hence there could be overrepresentation of women with maternal complications, our findings still warrant the importance of supporting modern family planning methods as a measure to improve IPI and thereby improve maternal outcomes as well.