The 30-year-old produced the maximum, which was the 100th in professional competition, in frame seven of his best-of-17 clash with Walden at the Barbican Centre in York.
Defending champion Selby came up with a brilliant shot off three cushions to get position from brown to blue, before taking a tricky pink on the stretch.
Despite leaving the white tight on the top cushion, he was able to drop the black into the centre pocket to complete the second 147 of his professional career.
Selby was rewarded with a cheque for £55,000 and will take home a further £4,000 for making the highest break of the tournament, providing that his achievement is not matched tonight or tomorrow.
The Leicester potter leads 6-2 going into tonight's final session. World number one Neil Robertson awaits the winner in Sunday's final.