A method for purifying a viral vector from a solution including the viral vector and host cell proteins (HCP) is provided. The method includes circulating the solution across an ultrafiltration/diafiltration membrane using tangential flow filtration (TFF) mode with continuous addition of diafiltration buffer. The method further includes filtering the solution across the ultrafiltration/diafiltration membrane to provide a permeate and a retentate. The retentate is collected such that a purified viral vector solution is obtained and retained in the retentate. A volume of the retentate is kept constant by the continuous addition of diafiltration buffer and the HCP is filtered out via the permeate.