We present the luminosity function (LF) for ultraluminous Lya emitting galaxies (LAEs) at z=6.6, defining ultraluminous LAEs (ULLAEs) as galaxies with log L (Lya)>43.5 erg/s. We selected candidates based on color cuts (g',r',i'>26, NB921<23.5, and NB921-z'<1.3) and visual inspection of a 30 sq. degree Subaru Hyper Suprime-Cam broadband (grizy) and narrowband (NB816 and NB921) survey.We followed up 14 candidates spectroscopically with DEIMOS on Keck II, confirming 9 candidates as ULLAEs at z=6.6. We also found 2 [OIII]5007 emitters at z ~ 0.84, a single AGN at z=6.6, and two spectroscopic non-detections. We constructed an ULLAE Luminosity Function from the 9 ULLAEs along with two previously confirmed ULLAEs in the COSMOS field: CR7 and COLA1. We applied a comprehensive incompleteness correction based on simulations of over 27 million test sources. In comparing our ULLAE LF at z=6.6 to LFs from the literature at z=6.6 and z=5.7, our data reject some previous LF normalizations but are broadly consistent with others. None of the current studies are fully consistent with one another, prompting the need for further study of the z=5.7 ULLAE LF using a consistent methodology in order to determine the evolution of the LF from z=6.6 to z=5.7.