Officers received reports of a two-vehicle collision involving a white Vauxhall Vivaro and a silver Renault Laguna just before 9.30am.
The male driver of the Renault, a man in his 60s, was taken to hospital, where he died some time later.
The crossroads junction on West End road with B1397 and Middlegate Road west were closed just after 12pm to allow emergency services to deal with the incident.
If you witnessed the collision or have any dashcam footage available, get in touch with Lincolnshire Police by calling 101, quoting incident 127 of 20 August, or email [email protected] quoting incident 127 of 20 August in the subject line.
If you wish to remain anonymous, you can also report any information via the independent charity CrimeStoppers on 0800 555 111 or online.