PurposeIn daily practice, oncologists and nurses frequently need to decide whether or not to refer a patient for professional mental health care. We explored the indicators oncologists and nurses use to judge the need for professional mental health care in patients with cancer.MethodsIn a qualitative study, oncologists (n= 8) and nurses (n= 6) were each asked to select patients who were or were not referred for professional mental health care (totaln= 75). During a semi-structured interview, they reflected on their decision concerning the possible referral of the patient. Thematic analysis was used to analyze the data.ResultsRespondents reported using a strategy when judging whether professional mental health care was needed. They allowed patients time to adjust, while monitoring patients’ psychological well-being, especially if patients exhibited specific risk factors. Risk and protective factors for emotional problems included personal, social, and disease- and treatment-related factors. Respondents considered referral for professional mental health care when they noted specific indicators of emotional problems. These indicators included lingering or increasing emotions, a disproportionate intensity of emotions, and emotions with a negative impact on a patient’s daily life or treatment.ConclusionsThis study identified the strategy, risk and protective factors, and the indicators of emotional problems used by oncologists and nurses when judging the need for professional mental health care in patients with cancer.Implications for Cancer SurvivorsOncologists and nurses can play an important role in the identification of patients in need of professional mental health care.Supplementary InformationThe online version contains supplementary material available at 10.1007/s11764-021-01151-2.