BK polyomavirus (BKPyV) is responsible for hemorrhagic cystitis and nephropathy in bone marrow- and kidney transplant recipients, respectively. Monitoring BKPyV DNAuria and/or DNAemia by quantitative molecular tests is thus a common practice in the management of these patients. However, critical issues have been raised regarding these methods and the detection of viral genomes by positive PCR is not always associated with true viral replication. Identification of novel biomarkers and development of new assays are thus needed to help clinicians in patients' follow-up and enable timely and beneficial therapeutic interventions. Here, we describe the development of a urinary lateral flow assay that specifically detects the major BKPyV VP1 protein in a few minutes. This assay was characterized using cell culture-produced virus, pseudoviruses as well as VP1 pentamers representing the four BKPyV genotypes, diluted in urine matrix. The limit of detection was around 1 µg/mL of purified VP1 pentamers. This assay also enabled to detect BKPyV VP1 in frozen urine from transplant patients with high DNAuria. Clinical validation studies are now needed to evaluate the performance of this assay in real life and its relevance for interventions.