De Minaur, hoping to repeat his 2024 championship win here, was the top seed left in the draw after Majchrzak's shock victory on Friday against world number four Felix Auger-Aliassime.
Majchrzak, the world number 76, backed up the best win of his career with another upset as he ground down Medvedev in Saturday's second semi-final.
The Pole claimed the opening set tie-break when Medvedev dumped a backhand into the net, his 23rd unforced error of the match.
Majchrzak secured the first break of the match early in the second set, sending a blistering backhand pass up the line.
An insurance break quickly followed as Majchrzak drilled a return at Medvedev's feet.
Medvedev then contrived to serve three consecutive double faults in his next service game to hand the second set to the 30-year-old Pole.
"It's not going to get any easier," said Majchrzak, looking ahead to Sunday's final.
"Alex is one of the greatest players, and obviously, he is playing well here on grass. I have played him twice and I've lost twice."
Check out the full match statistics on Flashscore
