Intrinsic or acquired resistance to chemotherapy is a major hurdle in the treatment of cancer. One of the key mechanisms of resistance is the overexpression of the drug efflux transporter P-glycoprotein (Pgp). Pgp overexpression renders a large number of mechanistically unrelated chemotherapies ineffective. Targeting Pgp inhibition directly to overcome drug resistance, although conceptually and mechanistically attractive, has not translated to the clinic, in part because Pgp also has a critical protective function in many healthy tissues. It was recently discovered that carbonic anhydrase XII (CA XII), an enzyme associated with pH regulation in cancer, is co-expressed and co-located with Pgp in drug resistant cancer cells. CA XII is also upregulated by hypoxia, which is another microenvironmental factor that contributes to drug resistance. Here, we review findings that demonstrate modulation of CA XII may offer a promising new approach towards overcoming the longstanding hurdle of drug resistance and therapy failure against solid cancers. This review covers the use of CA XII inhibitors, both small molecule and antibody, in combination with chemotherapeutics that are substrates for Pgp. This combination therapy approach restores the efficacy of chemotherapy in resistant cells and offers a potential new therapeutic window to re-examine the targeting of Pgp as a safe, effective, and novel anticancer strategy.