Vegetables are largely cultivated by smallholders and contribute immensely on agricultural growth in Kenya. Tomato is among vibrant vegetables that sustain rural households underwriting 7 percent of horticultural yield and 14 percent of vegetable output. However, tomato marketing experiences inefficiencies leading to unprecedented inequalities among smallholders. This is attributed to limited information on market expansion at farm level and inadequate literature on characteristics that affect market diversity. The study assessed available market channels and computed the magnitude of market expansion using the Simpsons index while Tobit regression model was applied to investigate factors influencing market expansion. Majority of the farmers were found to market their produce in at least three channels with prices across outlets differing significantly at 1 percent level. A large proportion of fresh tomatoes was sold through middlemen with contract markets recording the least despite attracting high produce prices. Besides, results show an average index of 0.4771 and showed that market diversity among smallholder farmers was responsive to household size, type of road, income, extension, experience, group membership, financial credit and distance to the nearest markets. This articulates an opportunity to diversify further thus enabling farmers to explore existing markets. Besides, farmers will be able to generate income from different channels and minimize losses that result from tomato spoilage during a glut. The study endorses policy interventions aimed at improving road infrastructures, promote organized groups, increased extension access and enhanced contract markets that reciprocate the benefits of market diversity.