Current knowledge on product marketing in Ethiopia is poor and inadequate for designing and implementing policies to overcome problems in the marketing system. This study was conducted to identify the factors influencing the market participation decision of smallholder haricot bean (Phaseolus vulgaris L.) farmers in Northwest Ethiopia. Survey data were collected from 312 smallholder farmers and analyzed using Heckman’s two-step econometric model that estimates probit model in the first step and regression model with the parameters estimated using the Ordinary Least Squares (OLS) method in the second step. The educational status, non-farm income from nonfarm employment, number of extension contacts, gender, improved seed use, chemical fertilizer, and farmers' perception of land degradation were the significant variables affecting the market participation decision of smallholder farmers. The amount of haricot bean output supplied to the market were influenced by age, experience, livestock holding, nonfarm income, extension contacts, gender, market access, and membership in marketing association. Participation in the market can be improved by providing training and education regarding the production and marketing of haricot bean output and increasing farmers' contact with extension agents.