People frequently participate in interdependent tasks (i.e., tasks in which the outcome of one person is reliant on the other person's actions), in which people can behave in ways that benefit others (i.e., cooperate) to achieve mutually beneficial outcomes in daily life. The ability to select appropriate cooperative partners for these tasks is essential to achieve successful outcomes. Yet, little is known about individual partner preferences for interdependent tasks and whether these preferences change in response to situational affordances of the task (i.e., which traits can affect task outcomes). Here, we report four studies (N = 1021) that investigate the relationship between partner preference, person perceptions, and partner selection in interdependent tasks that afford the expression of warmth- or competence-related traits to affect outcomes. Over four studies, participants were randomly assigned to an interdependent task affording for warmth- or competence-related traits, then rated the most important traits in a partner (Study 1–4), evaluated potential partners' warmth and competence (Study 3–4), and selected partners (Study 3–4). Overall, participants strongly prefer warmth-related traits in a partner, but partner preferences also vary depending on task affordance. Specifically, people demonstrated a stronger preference for partner trustworthiness in tasks affording warmth-related traits and preferred highly competent partners in tasks affording competence-related traits. Additionally, preferences for partner traits strengthened the relationship between the perceived partner trait afforded by the situation and partner selection. We discuss these findings in relation to theories of partner selection and cooperation, as well as the implications of these results to develop tools and interventions to help people optimize their partner selections.