The collision involving a green Honda motorcycle happened at around 12.20pm on Sunday (September 11) at the roundabout junction with Parklands and the A428 near Crick.
As a result of the collision, the rider — a man in his 30s from Warwickshire — sustained serious injuries and was taken to University Hospital Coventry, where he sadly died on Monday.
Crash investigators believe the bike hit a kerb although why it did remains a puzzle.
A Northamptonshire Police spokesman said: “Investigations into the collision continue and officers would like to hear from anyone who may have witnessed the incident or who may have dash-cam footage of the actual collision or the motorcycle prior to the incident.
“Anyone with information can email CollisionApp[email protected], call the Drivewatch Hotline on 0800 174615 or contact Northamptonshire Police on 101 using incident number: 22000529718.”
Sunday’s crash is now the second fatal incident on the A5 in Northampton in less than 72 hours.
Police are still appealing for witnesses after a woman passenger died and a driver seriously injured when a black Nissan Note was in a collision with a white Jaguar Xe Prestige at the crossroads near Whilton at around 1.40pm on Thursday (September 8).