Bruton's tyrosine kinase (BTK) is a major drug target in immune cells. The membrane-binding pleckstrin homology and tec homology (PH-TH) domains of BTK are required for signaling. Dimerization of the PH-TH module strongly stimulates the kinase activity of BTK in vitro. Here, we investigated whether BTK dimerizes in cells using the PH-TH module and whether this dimerization is necessary for signaling. To address this question, we developed high-throughput mutagenesis assays for BTK function in Ramos B cells and Jurkat T cells. We measured the fitness costs for thousands of point mutations in the PH-TH module and kinase domain to assess whether dimerization of the PH-TH module and BTK kinase activity were necessary for function. In Ramos cells, we found that neither PH-TH dimerization nor kinase activity was required for BTK signaling. Instead, in Ramos cells, BTK signaling was enhanced by PH-TH module mutations that increased membrane adsorption, even at the cost of reduced PH-TH dimerization. In contrast, in Jurkat cells, we found that BTK signaling depended on both PH-TH dimerization and kinase activity. Evolutionary analysis indicated that BTK proteins in organisms that evolved before the divergence of ray-finned fishes lacked PH-TH dimerization but had active kinase domains, similar to other Tec family kinases. Thus, PH-TH dimerization is a distinct feature of BTK that evolved to exert stricter regulatory control on kinase activity as adaptive immune systems gained increased complexity.