Valensi's beloved orange Epiphone recently went missing while it was being shipped between New York and Los Angeles, and the stars reached out to fans earlier this month (Jan16), asking them to stay alert and pass on any information that could help the musician locate it.
The instrument has yet to turn up, and now The Strokes have upped the reward from $1,000 (£670) to $5,000 (£3,330) in a desperate bid to get it back.
"Unfortunately, we still haven't found Nick's missing guitar," the stars write on Twitter.com. "we've updated the amount on the reward we are offering".
The Strokes are believed to be working on a new album, two years after their last project, 2013's Comedown MAChine, after playing a handful of dates together in recent months.
In the original appeal posted on the band's Facebook.com page, the rockers wrote, "Hello Friends, In a bit of unfortunate news, we've recently learned that Nick's beloved Epiphone guitar has gone missing while being shipped from NYC to LA...
"We ask that you join us in keeping an eye out for it. It goes without saying how bummed he is that it's gone missing. If you have any info at all about the guitar - perhaps see it in a pawn shop or on eBay - please send an email... We deeply appreciate the extra eyes and ears."