The last time Liverpool were 3-0 down at half time in the Champions League was in 2014 to Real Madrid
Jurgen Klopp's side were 3-0 down at half time but it could easily have been more as Alisson saved a penalty and Virgil van Dijk made a goalline block.
Piotr Zielinski put Napoli ahead from the spot in the fifth minute after Victor Osimhen had hit the post.
Andre-Frank Anguissa and Giovanni Simeone extended Napoli's lead, before Zielinski scored again after the break.
It was a terrible opening half from Liverpool as Joe Gomez struggled to deal with Osimhen and was relieved when the Napoli forward trudged off with an injury before half-time.
His replacement, Simeone, needed just three minutes to score on his Champions League debut as Liverpool looked shell-shocked by the home side's relentless forward approach.
There were chances for the visitors - Van Dijk had a header well saved, Mohamed Salah struck an effort at the goalkeeper and a teasing cross bounced dangerously off Harvey Elliott's knee at the back post.
But Liverpool were sloppy in possession - even Salah mis-controlled a good pass from Trent Alexander-Arnold when well-placed in the box - and their defensive mistakes were punished by an expressive Napoli side.
Any hope of a second-half comeback were over when Zielinski chipped a finish over Alisson from close range to make it 4-0 and though Luis Diaz curled in for an immediate response and came close with a diving header, Liverpool were deservedly beaten.
Deadline-day signing Arthur Melo came on for his debut late in the second half and fellow midfielder Thiago returned from injury to make a brief cameo, but Liverpool will head home having now won just two of their last seven matches in all competitions.
From the very first minute, Liverpool were on the back foot in Naples.
Osimhen gave them an early warning when he rounded Alisson and fired against the post from a tight angle.
Napoli, though, needed only five minutes to take the lead when Zielinski coolly slotted in his penalty after handball against James Milner.
A typical Klopp side would have been expected to bounce back immediately, but this season Liverpool have looked tired, demotivated and seriously out of form.
What followed instead was only humiliation as the hosts were ruthless, while Liverpool made numerous mistakes and struggled to get a stronghold of the game with talisman Salah having several poor touches.
Osimhen ran the struggling Gomez ragged and Napoli could have scored at least another two goals, were it not for a goalline clearance from Van Dijk to deny Khvicha Kvaratskhelia and a penalty save from Allison to thwart Osimhen after the lively frontman had been caught by Van Dijk.
Former Fulham midfielder Anguissa doubled Napoli's lead in style as a quick one-two ripped apart Liverpool's defence, before Kvaratskhelia set up Simeone's third.
When the whistle finally blew for the break, Klopp sprinted straight down the tunnel.
The last time Liverpool conceded three goals in the first half of a Champions League game was in October 2014 when they lost 3-0 to Real Madrid.
"That's an embarrassing 45 minutes," former Manchester United defender Rio Ferdinand said on BT Sport,
Liverpool at least improved in the second half but Napoli remained in complete control.
The impressive Zielinski added the fourth two minutes after the restart and Diaz's curling effort in the 49th minute was no consolation for the disappointing six-time European champions.
-- Courtesy of BBC Sport