de Angelis, like reigning double 250cc world champion Jorge Lorenzo and Andrea Dovizioso, will graduate to MotoGP next season - but KTM star Kallio outshone them all on Sunday.
Lorenzo, who has taken nine race wins this season, began the race from pole and took an early lead - but failed to pull away and surprisingly began to plummet down the order. Valentino Rossi's 2008 Fiat Yamaha team-mate eventually finished a forgettable seventh.
de Angelis had replaced Lorenzo at the front on lap 10, and pulled a lead through the middle stages, but was then reeled back in by Kallio and home hero Alvaro Bautista. Bautista subsequently fell, but Kallio remained on target and passed de Angelis - whose only grand prix win came at Valencia last year - on the penultimate lap.
The Finn went on to take victory by 0.371secs, while Lorenzo's Fortuna Aprilia team-mate, and 2008 replacement, Alex Debon won a close duel with Dovizioso for the final podium position. An angry Dovi felt that Debon squeezed him on the run to the flag, as the Spaniard edged the Honda-mounted Italian by just 0.083secs.
Race results – Valencia:
1. Kallio (KTM)
2. de Angelis (Aprilia) +0.371secs
3. Debon (Aprilia) +6.797secs
4. Dovizioso (Honda) +6.880secs
5. Barbera (Aprilia) +12.767secs
6. Simon (Honda) +13.030secs
7. Lorenzo (Aprilia) +14.751secs
8. Takahashi (Honda) +16.437secs
9. Luthi (Aprilia) +16.551secs
10. H. Aoyama (KTM) +20.223secs
21. Laverty (Honda) +1min 27.529secs