This is a research study to test the safety of the investigational combination of tazemetostat and venetoclax for patients with non-Hodgkin lymphoma (NHL) whose disease has returned after having received prior treatment (relapsed), or whose disease has failed to respond to prior treatments (refractory).

This study will find the highest dose of venetoclax that can be given with tazemetostat without causing severe side effects. Both of these medications have been studied in patients with lymphoma and have been shown to be safe, however, they have not been used in combination.

All participants will receive a combination of oral 800 mg tazemetostat and oral venetoclax. The dose of venetoclax will be determined by the study team.

Participants will need to regularly come to the clinic for blood work, imaging, and to monitor any side effects. Participants may receive study drug until their cancer progresses or for up to 24 months.