The Diamonds will travel to Northampton to face defending champions Southern Vipers in a repeat of last season’s final.
Beth Langston, Rachel Slater and Linsey Smith claimed two wickets before 42 apiece for Gwen Davies and Ria Fackrell recovered the Sparks from 68-5 to 175-7.
The Diamonds fell from 33-0 to 42-3 in reply before aggressive Ami Campbell and more reserved Dutch international Sterre Kalis shared 116 for the fourth wicket, posting 76 off 78 balls and 42 not out respectively.
Campbell’s half-century was her first of the season as the six-wicket win was secured with 17.1 overs remaining.
“It just feels great to have come off the back of a fairly poor season for me and to do something good,” said Campbell.
“To get us over the line and into the final is superb.
“I felt very nervous when I went in. I knew I had to stay there and was quite defensive at the start. That’s not my usual game, but it’s something I’m getting better at and am working on.”
Sparks captain Eve Jones was one of three wickets to fall inside 10 overs – 35-3 – after miscuing a cut at Slater’s left-arm seam.
Marie Kelly and Issy Wong were both trapped lbw for golden ducks, the latter as one of two wickets in three balls for left-arm spinner Smith as the score fell to 68-5.
Davies and Fackrell both played in reserved fashion in their attempt to turn the tide.
And while they managed to gain respectability, with the help of Emily Arlott’s 29, it proved to be nowhere near enough against a Diamonds side whose only blemish in the first half of the game was a couple of dropped catches.
Langston and Smith returned 2-29 and 2-25 from their 10 overs.
Captain Hollie Armitage, whose side has now reached all three finals in the regional era –they are still chasing a first trophy – then got the chase off to a flying start with five boundaries in her first nine balls.
Campbell reached her fifty off 51 balls before falling with victory all but secured.