Nonadherence to immunosuppressive medications after kidney transplantation is a major risk factor for allograft rejection and graft loss. Transplant recipients tend to conceal nonadherence in clinical settings. We conducted an internet-based cross-sectional survey aimed to clarify the prevalence and risk factors of patient nonadherence after kidney transplantation in Japan. The Basel Assessment of Adherence to Immunosuppressive Medication Scale (BAASIS©) was used to detect nonadherence. In this survey, patients were asked about their relationship with medical staff and their self-efficacy for taking medication, along with other questions. A total of 219 kidney transplant recipients were included in the analysis. Ninety-four demonstrated nonadherence (42.7%). The following factors were associated with nonadherence: short dialysis period prior to transplantation; undergoing transplantation at least twice; male sex; lack of satisfaction with the explanation of immunosuppressive medication; and absence of medical staff to consult upon forgetting to take drugs. The nonadherence group reported low self-efficacy for medication in many settings. This anonymous survey provides valuable insight into the actual nonadherence rate, factors associated with nonadherence, and life situations that may complicate medication-taking.