The 6-4, 6-3 victory will take De Minaur to a career-high number nine in the world rankings - his first time in the top 10 - and sets up a possible final with Australian Open champion Jannik Sinner.
It was deja vu for the Australian - he also spent his 24th birthday in Rotterdam playing Dimitrov. Last year, de Minaur squandered two match points to crash out in the quarters. This time, he started quickly, breaking Dimitrov's first service game to love.
Dimitrov worked his way into the match using his trademark backhand slice to break up the Australian's rhythm and powerful forehand groundstrokes to back up a strong service.
De Minaur retrieved impressively and made few unforced errors, holding his serve despite constant Dimitrov pressure and eventually taking an entertaining first set 6-4.
In the second set, Dimitrov again failed to hold his first service game, handing the initiative immediately to the Australian.
The Rotterdam crowd was treated to some spectacular rallies as De Minaur found answers to everything Dimitrov threw at him, closing out the second set 6-3.
"I feel amazing. This year was a better birthday than I had last year," he said after the match.
"Grigor is so talented, such a good player. Today I had to be basically flawless to get the win over him. On to the final, so happy days."
Saturday's second semi-final sees Australian Open champion Sinner seek to extend his 2024 unbeaten run against home favourite Tallon Griekspoor.
The Italian, who is on a 13-match winning streak, can climb to a career-high third in the world rankings with a win in Rotterdam. He came through his quarter-final after Canadian veteran Milos Raonic withdrew injured. Griekspoor has never beaten anyone in the top five but can count on raucous home support.
The match is a repeat of last year's semi-final which Sinner took in two close sets.
