This study assessed sanitation access in rapidly expanding informal settlements in Dar es Salaam (Dar) against eight proposed indicators of hygienic safety, sustainability and functionality, and in relation to the Millennium Development Goal (MDG) ‘improved’ sanitation definition. Information was collected on toilet facility designs, management and functionality through a structured interview and observations at 662 randomly selected residential properties across 35 unplanned, low-income sub-wards of Dar. Trends in access and associations with sharing, occupancy, latrine replacement, income, education and location factors were considered through statistical analyses. Surveyed sub-wards were open-defecation free. While 56% of households used a facility that met the MDG improved technology definition, only 8% had a functional facility that could be considered as hygienically safe and sustainable sanitation. Safe, sustainable, functioning sanitation access was 2.6 times greater among the richest quintile than the two poorest quintiles. Very poor sanitation services among Dar's urban poor arise from widespread lack of access to hygienically safe pit emptying services, unhygienic designs and functionality problems (affecting 67, 55 and 29%, respectively). As new goals and targets beyond 2015 are discussed, these findings may have important implications for defining what constitutes ‘improved’ sanitation for poor populations living in unplanned informal settlements.