Tankyrase1 is a multifunctional poly(ADP-ribose) polymerase that can localize to telomeres through its interaction with the shelterin component TRF1. Tankyrase1 poly(ADP-ribosyl)ates TRF1 in vitro, and its nuclear overexpression leads to loss of TRF1 and telomere elongation, suggesting that tankyrase1 is a positive regulator of telomere length. In agreement with this proposal, we show that tankyrase1 RNA interference results in telomere shortening proportional to the level of knockdown. Furthermore, we show that a tankyrase1-resistant form of TRF1 enforced normal telomere length control, indicating that tankyrase1 is not required downstream of TRF1 in this pathway. Thus, in human cells, tankyrase1 appears to act upstream of TRF1, promoting telomere elongation through the removal of TRF1. This pathway appears absent from mouse cells. We show that murine TRF1, which lacks the canonical tankyrase1-binding site, is not a substrate for tankyrase1 poly(ADP-ribosyl)sylation in vitro. Furthermore, overexpression of tankyrase1 in mouse nuclei did not remove TRF1 from telomeres and had no detectable effect on other components of mouse shelterin. We propose that the tankyrase1-controlled telomere extension is a human-specific elaboration that allows additional control over telomere length in telomerase positive cells.