The 36-year-old overhauled Dan Martin in the final 300 metres to add to his fifth win at Fleche Wallonne on Wednesday.
The 36-year-old overhauled Irishman Dan Martin in the final 300 metres to add to his fifth win at Fleche Wallonne on Wednesday.
"This win is for Michele Scarponi. He was a good friend of mine," said an emotional Valverde.
Italian Scarponi died in a training accident after being struck by a van in central Italy on Saturday.
"It hurts to think he's gone, I'll miss him. I'll give all my prize money from this victory to his family. This win is also dedicated to him."
It has been a remarkable start to the season for Valverde, who turns 37 on Tuesday.
He has won four stage races -- the tours of Catalonia, the Basque Country, Andalusia and his home region of Murcia -- already this season, and two one-day classics.
On Wednesday he extended his record of Fleche Wallonne successes to five and has now added a 'Monument' victory to his remarkable accolades.
Only the great Eddy Merckx, with five, has won Liege more times than Valverde.
Victory in the epic 258km race came 11 years after Valverde first tasted success at Liege.
The day started with a minute's applause in memory of Scarponi.
An eight-man breakaway escaped after 7km and opened up a lead of 13 minutes at one stage but its final remnant, Stephane Rossetto held on until the final 6km.
Further attacks followed before Italian Davide Formolo broke away on his own.
As he tired on the final climb up to the finish at Ans, just outside Liege, Martin set off after him.
But the Irishman tied up and Valverde caught him in the final straight before powering clear to win, consigning Martin to the runner-up spot for the second time in four days.
"On the last climb Dan Martin attacked really hard but I managed to reach him at the right moment to then sprint to the finish line," added Valverde.
Poland's Michal Kwiatkowski won the sprint for third ahead of Australia's Michael Matthews three seconds back.
Martin, who quipped on Wednesday he might have to wait for Valverde to retire before winning Fleche, was gracious in defeat again.
"I was on the edge with the belief that I could do it but Alejandro came back to me," he said.
"I'm certainly not frustrated to have been beaten by a rider who was stronger than me."
Earlier in the day, Olympic champion Anna van der Breggen completed the treble of Ardennes Classics following victories over the last week at Amstel Gold and La Fleche Wallonne by winning the first ever edition of the women's race, over 135km.