Purpose: To develop a physical function test based on lie-to-sit transition and to study its feasibility in patients suffering from metastatic breast cancer (MBC). Materials and methods: This cross-sectional study recruited 90 women diagnosed with MBC. Patients were asked to transfer from lying to sitting position as fast as possible during 30 s, performing the 30-second lie-to-sit test (30-LTS). Heart rate (HR), rate of perceived exertion (RPE) and number of repetitions were measured. An assessment included the 30-second sit-to-stand test (30-STS), handgrip strength, Upper Limb Functional Index (ULFI) and Lower Limb Functional Index (LLFI). Pearson correlation was calculated between 30-LTS and independent outcomes. A linear regression model explaining the 30-LTS results was further constructed with variables that had a significant correlation. Results: About 72 patients were measured, of which 65 were able to perform 30-LTS. Subjects performed 8.13 repetitions on average, with a mean RPE of 4.78 (0–10), reaching 63.08% of maximal HR. 30-LTS was significantly correlated with 30-STS (r = 0.567), handgrip (p = 0.26) and LLFI (r = 0.348). The regression model was significant (F = 4.742; p = 0.00), and these variables explained 32% of the variance of the 30-LTS. Conclusion: The 30-LTS showed to be a feasible functional and submaximal test in a sample of MBC. IMPLICATIONS FOR REHABILITATION The 30-second lie-to-sit (30-LTS) developed does not require the patient to acquire a standing position and therefore it is an alternative to other more biomechanically demanding tests such as a 30 second sit-to-stand test or Timed up-and-go. 30-LTS involves both a functional and energy system assessment tool that can be implemented by allied health professionals in oncology rehabilitation to individualize exercise prescription, as well as for functional screening purposes. The present study adds value to current research focused on individualizing exercise prescription in the oncology field and provides reference values of function in metastatic breast cancer patients.