Santiago Botero suffered on the climb
Jimenez made the decisive move seven kilometres from the end of the 195km-stage on the first category climb to Alto de la Demanda.
Joseba Beloki broke away from a 15-man chasing group to take second place 22 seconds behind Jimenez and claim the overall lead.
Previous leader Oscar Sevilla had been in the chasing group but fell back during the climb and finished 49 seconds back in 11th place.
Carlos Sastre of the ONCE team had looked the most likely to catch Jimenez, who happens to be his brother-in-law, but could not stand the pace and slipped back into the pack.
Festina's Angel Casero team moved up into third place ahead of his Festina team-mate David Plaza.
Kelme's Santiago Botero suffered on the final ascent and dropped to fifth place overall.
Sunday's ninth stage should favour the sprinters as it is a straightforward 179.2-km run from Logrono to Zaragoza.
Tour of Spain stage eight results:
1. Jose Maria Jimenez, Spain, ibanesto.com, 4 hours 43
minutes, 2 seconds
2. Joseba Beloki, Spain, ONCE, 22 seconds behind
3. Juan Miguel Mercado, Spain, Banesto, 27 secs
4. Levy Leipheimer, United States, US Postal, 29 secs
5. Roberto Laiseka, Spain, Euskatel, same time
6. Manuel Beltran, Spain, Mapei, 46 secs
7. Iban Mayo, Spain, Euskatel, 47 secs
8. Inigo Cuesta, Spain, Cofidis, 56 secs
9. Roberto Heras, Spain, US Postal, 59 secs
10. Angel Casero, Spain, Festina, 1min 1sec behind
Overall standings:
1. Joseba Beloki, Spain, ONCE, 25 hours, 6 minutes, 17
seconds
2. Oscar Sevilla, Spain, Kelme, 15 seconds behind
3. Angel Casero, Spain, Festina, 51 secs
4. David Plaza, Spain, Festina, 1min 27secs
5. Santiago Botero, Colombia, Kelme, 1min 28secs
6. Levy Leipheimer, United States, US Postal, 1min 54secs
7. Juan Miguel Mercado, Spain, Banesto, 2mins
8. Roberto Heras, Spain, US Postal, 2mins 47secs
9. Iban Mayo, Spain, Euskatel, 3mins 14secs
10. Igor Gonzalez Galdeano, Spain, ONCE, 3mins 29secs