<p dir="ltr">Within the last decade, cancer immunotherapy, the therapeutic strategy of enhancing the body’s immune system to curb tumor growth, has reached the front lines in the war on cancer. Although common strategies such as adoptive cell transfer and immune checkpoint blockade have enjoyed success against some cancers, they regrettably lack durable efficacy across a broad patient population inflicted by heterogeneous and diverse cancer types. Moreover, application of these biological therapeutics is likewise limited due to various toxicities frequently encountered in the clinic. Taking these into account, the next generation of immunotherapies must exploit novel immunomodulatory targets and therapeutic strategies that can possess both enhanced efficacy compared to current options and more acceptable toxicity profiles in patients. Compared to biologics, small molecule inhibitors are desirable as they may circumvent concerns involving efficacy and toxicity, while allowing access to a broader arsenal of macromolecular targets. Recently, protein tyrosine phosphatase nonreceptor 22 (PTPN22), a key desensitization node in T cell signaling, has emerged as a systemic and translatable cancer immunotherapy target. Nonetheless, many of its precise functions in various immune cells is not fully resolved, thus there is a critical need for both novel chemical probes for biological interrogation and inhibitors with improved <i>in vivo </i>efficacy for further therapeutic development.</p><p dir="ltr">Built upon an overview of PTPN22’s structure, function, and value as an immunotherapy target, as well as a comprehensive assessment of reported inhibitors, this dissertation documents two separate medicinal chemistry campaigns on existing PTPN22 scaffolds. Herein, the structure activity relationships, design, and biological evaluation of a novel, superiorly selective and cell-active probe/ lead compound is disclosed. This dissertation also reports the design of a novel PTPN22 inhibitor with enhanced potency, selectivity, cellular efficacy, <i>in vivo </i>pharmacokinetics, and <i>in vivo </i>antitumor efficacy in mice. Our research efforts and the overall status and future directions of the field are also succinctly discussed.</p>