Lymphoplasmacytic Lymphoma (LPL) is a rare type of low-grade B-cell lymphoma. The purpose of this study is to assess the change in disease activity of adult participants with relapsed or refractory Waldenström macroglobulinemia(WM)/LPL receiving venetoclax. Venetoclax is being investigated in the treatment of WM/LPL. Participants will receive oral venetoclax at doses ramping up to the target dose, as part of treatment. Approximately 14 adult participants with WM/LPL will be enrolled in the study at approximately 20 sites in Japan. Participants will receive oral venetoclax at doses ramping up to the target dose. The total study duration is approximately 28 months. There may be higher treatment burden for participants in this trial compared to their standard of care. Participants will attend regular visits during the study at a hospital or clinic. The effect of the treatment will be checked by medical assessments, blood tests, and checking for side effects.