The Movistar rider took second place in the final time trial behind Slovenia's Primoz Roglic to add to his Tour of Murcia, Tour of Andalucia and Tour of Catalonia titles this season.
Valverde, one of very few riders with major titles or podium finishes in both grand tours and one-day races, controlled the opposition in the mountains before securing the win in the 27.7-km time trial around Eibar on Saturday.
Overall, he beat fellow Spaniards Alberto Contador and Jon Izaguirre by 17 and 21 seconds respectively.
The 36-year-old will be the man to watch in the Ardennes classics that start next weekend with the Amstel Gold Race in the Netherlands.
Two races in Belgium then follow -- the Fleche Wallonne, which Valverde has won for the last three years, and Liege-Bastogne-Liege, which he has claimed on four occasions.
Stage six result
1. Primoz Roglic (Slovenia / LottoNL) 35:58"
2. Alejandro Valverde (Spain / Movistar) +9"
3. Ion Izagirre (Spain / Bahrain) +15"
4. Alberto Contador (Spain / Trek) +23"
5. David De La Cruz (Spain / Quick-Step) +34"
6. Michael Matthews (Australia / Sunweb) +41"
7. Vasil Kiryienka (Belarus / Team Sky) +52"
8. Diego Ulissi (Italy / UAE Team Emirates)
9. Victor De la Parte (Spain / Movistar) +1:04"
10. George Bennett (New Zealand / LottoNL) +1:23"
1. Alejandro Valverde (Spain / Movistar) 20:41:25"
2. Alberto Contador (Spain / Trek) +17"
3. Ion Izagirre (Spain / Bahrain) +21"
4. David De La Cruz (Spain / Quick-Step) +44"
5. Primoz Roglic (Slovenia / LottoNL) +59"
6. Louis Meintjes (South Africa / UAE Team Emirates) +1:19"
7. Patrick Konrad (Austria / BORA) +1:40"
8. Sergio Henao (Colombia / Team Sky) +1:51"
9. Rigoberto Uran (Colombia / Cannondale) +1:56"
10. Simon Spilak (Slovenia / Katusha) +2:01"
Read the original article on Eurosport: In-form Valverde wins Tour of Basque country