Abstract Telomere length maintenance is crucial for cells that divide many times. TIN2 is an important regulator of telomere length, and mutations in TINF2 , the gene encoding TIN2, cause short telomere syndromes. While the genetics underscore the importance of TIN2, the mechanism through which TIN2 regulates telomere length remains unclear. Here, we characterize the effects of TIN2 on telomerase activity. We identified a new isoform in human cells, TIN2M, that is expressed at similar levels to previously studied TIN2 isoforms. Additionally, we found that all three TIN2 isoforms stimulated telomerase processivity beyond the previously characterized stimulation by TPP1/POT1. Mutations in the TPP1 TEL-patch abrogated this stimulation, implicating TIN2 as a component of the TPP1/POT1 processivity complex. All three TIN2 isoforms localized to telomeres in vivo but had distinct effects on telomere length, suggesting they are functionally distinct. These data contrast previous descriptions of TIN2 a simple scaffolding protein, showing that TIN2 isoforms directly regulate telomerase.