The 8-11 shot defeated 12-1 chance Algometer by one and a half lengths with the other four rivals well beaten.
Midterm is a son of 2001 Derby winner Galileo and Group One victor Midday.
After the Sandown win, Midterm's odds for the Derby at Epsom in June were cut by some bookmakers from 8-1 to 6-1 second favourite.
The 4-1 market leader is US Army Ranger.
In the 1980s, the trial was won by subsequent Derby winners Shergar and Shahrastani, although the last runner to triumph at Epsom was 1997 Sandown runner-up Benny The Dip.
You can check out the latest racing results on the BBC Sport website.
Odds-on favourite Midterm, ridden by Ryan Moore for trainer Sir Michael Stoute, won the Classic Trial at Sandown on Friday afternoon.