Livestock is an important component of crop-livestock farming systems in Sub-Saharan Africa (SSA). This paper examined the effect of livestock on crop commercialization and poverty reduction among smallholder farmers in crop-livestock farming systems in Singida Region, Tanzania. It was hypothesized that livestock enhances crop commercialization and reduce poverty among smallholder farmers in the Region. Data for the analysis were extracted from the Agricultural Policy Research for Africa (APRA) data set of 600 households selected randomly from random samples of eight and seven villages in Iramba and Mkalama districts respectively. Descriptive statistics were used to compare ownership of livestock, use of ox-plough and livestock manure, crop productivity, crop commercialization and poverty levels across different categories of farmers. Econometric analyses were used to determine if livestock had a significant effect on crop commercialization and poverty levels, controlling for other variables that might have an effect. The results of descriptive analyses show differences in ownership of livestock, use of ox-plough and livestock manure, crop productivity, crop commercialization and poverty levels across different categories of farmers while the results of econometric analysis show that livestock enhanced crop commercialization. Apart from livestock, a range of other factors have worked together with livestock to drive the crop commercialization process. Regarding the impact of commercialization, the findings show that farmers have gained higher productivity (yield), signifying the potential of crop commercialization to reduce poverty. In general, evidence from the results show decline in poverty as crop commercialization increases from zero to medium level. Although crop commercialization has positively impacted on crop productivity (yields) and poverty, the results show existence of socio-economic disparities. Male-headed households (MHH) and households headed by medium-scale farmers (MSF), young farmers and livestock keepers were less poor than their counterpart female-headed households (FHH) and households headed by small-scale farmers (SSFs), older farmers and non-livestock keepers. These social differences are consequences of differences in the use of ox-plough, livestock manure and other productivity enhancing inputs. Exploiting the synergy between crop and livestock in crop-livestock farming systems needs to be recognized and exploited in efforts geared towards enhancing crop commercialization and reducing poverty among smallholder farmers in crop-livestock farming systems in Tanzania and elsewhere in SSA.