Hematopoietic progenitor kinase 1 (HPK1) induces potent anti-tumor immunity in preclinical models by activating and recruiting T cells, B cells, and dendritic cells into the tumor microenvironment (TME). Here, we evaluate NDI-101150, a potent, selective HPK1 inhibitor, in a phase 1/2 trial as a monotherapy or in combination with pembrolizumab in patients with advanced solid tumors. The monotherapy maximum tolerated dose (MTD) is 150 mg once daily, and doses tested up to 100 mg once daily are combinable with pembrolizumab without reaching an MTD. In clear cell renal cell carcinoma, the investigator-assessed overall response rate with monotherapy treatment is 13.6%, including one complete response and two partial responses, with a clinical benefit rate of 27.3% and a disease control rate of 54.5%. Pharmacodynamic analyses show pharmacodynamic biomarker phospho-SLP76 inhibition and increased activated CD8(+) T cells and dendritic cells in the TME, supporting continued development (clinical registration number NCT05128487).