The world champion attacked from a group of favourites with 54km remaining and never looked back. He swept up the remnants of the early break, with Silvan Dillier (AG2R la Mondiale) the only rider to capable of staying with him.
The duo worked together over the cobbles, and despite a late fightback from the chasers, entered the velodrome together. Sagan took the sprint to seal his second Monument win, and end Quick-Step's domination of the Spring Classics.
Dillier's classy ride was enough to seal second, with former winner Niki Terpstra (Quick-Step Floors) completing the podium. Last year's winner, Greg Van Avermaet (BMC Racing), who attacked just before Sagan, was forced to settle for fourth.
The day belonged to the world champion, who became the first rider in 37 years to win Paris-Roubaix in the rainbow jersey.
Sagan's winning attack came after excellent work from his teammates, who nullified several dangerous moves, and kept their leader clear of trouble when several other riders were caught behind in crashes.
His attack came at almost the same point at which Tom Boonen broke free to win in 2012. Considering Boonen's recent criticism over the Bora rider, Sagan could not have offered a more powerful response, letting his legs do the talking, as Boonen's former teammates forlornly tried to get on terms.
Fittingly, it was Dillier who entered the warm roar of the velodrome first. The Swiss champion was a late call-up for AG2R, and spent over 200km on the attack, but matched Sagan pedal stroke for pedal stroke when the Bora rider made contact. Dillier took to the high banking and while he clearly gave it one last effort, he was no match for Sagan, who raised his arms as he took the win.
"This is amazing," Sagan said in a post-race flash interview. "I have to say that this year, I was never involved in a crash, I never flat tired, I saved energy and then just did one step forward; I attacked. I kept going until the finish.
"I, now, stayed much better this year than in all the other years that I have finished Paris-Roubaix, where I was much more tired than today.
"I am very happy. I have to say thank you to all my teammates because they did a great job. They kept the group all together from the start. I did my winning move with 50km to go and I'm very happy to come first. It's an amazing feeling."
And as first and second free-wheeled around the velodrome to soak up the applause, Terpstra appeared from nowhere. The Tour of Flanders winner had been active once Sagan had attacked, and formed a dangers group alongside Van Avermaet, Sep Vanmarcke (EF Education First), Taylor Phinney (EF Education First), Jasper Stuyven (Trek Segafredo), Wout Van Aert (Veranda’s Willems Crelan), and Jens Debusschere (Lotto-Soudal).
The counter-attack brought Sagan to within 43 seconds at one point, but the world champion was in a league of his own, and once out of sight their next rendezvous came inside the velodrome.
How it unfolded
The 174 riders at the start in Compiègne were able to get ready for the 257-kilometre race while taking in a bit of sun. Meanwhile, in the north, rain was coming down on several pavé sectors. Tom Stamsnijder (Sunweb) was a non-starter.
As expected the race took a fast start out of Compiègne. Only after an hour, a breakaway move was able to distance the peloton.
The group existed of nine riders, being Sven Erik Bystrøm (UAE Team Emirates), Silvan Dillier (AG2R La Mondiale), Marc Soler (Movistar Team), Ludovic Robeet (WB Aqua Protect Veranclassic), Jimmy Duquennoy (WB Aqua Protect Veranclassic), Jelle Wallays (Lotto Soudal), Geoffrey Soupe (Cofidis, Solutions Credits), Gatis Smukulis (Delko Marseille Provence KTM), and Jay Robert Thomson (Dimension Data).
Their lead grew up to more than eight minutes in the long run-up to the cobbles. The speed picked up in the peloton when approaching the first pavé sector in Troisvilles after 165 kilometres. Just before this sector, there were two crashes that took out Stefan Küng (BMC) and Nelson Oliveira (Movistar).
Mud on the cobbles leads to crashes
The leaders hit the first cobbles at pavé sector 29 in Troisvilles with a lead of eight minutes on the peloton. In contrast to previous years, the cobbles were covered with mud. The leaders had no problems on this sector, but much later, the peloton didn't pass through unscathed.
There was a massive crash in the belly of the peloton with Mads Würtz Schmidt (Katusha) injured, but he was able to continue his race. Geraint Thomas (Team Sky) was one of the victims too, and he abandoned little later. Last year's winner Greg Van Avermaet (BMC), Belgian champion Oliver Naesen (AG2R) and in-form Yves Lampaert (Quick-Step Floors) were among the many distanced riders.
Katusha, Quick-Step Floors and Bora-Hansgrohe charged forward in the peloton, with the Van Avermaet group at half a minute.
On the following section Michael Goolaerts (Veranda's Willems-Crelan) crashed heavily. The first messages concerning his crash indicated he needed urgent help. NOS was able to contact team DS Michiel Elijzen who said Goolaerts had a cardiac arrest and received CPR. [ed update 22:57 BST: We are deeply saddened to write that Michael Goolaerts died this evening. Our thoughts are with his family, friends and teammates].
There were punctures from John Degenkolb, Dylan Groenewegen, Zdenek Stybar and Arnaud Démare. They were able to come back into the Quick-Step Floors controlled peloton at 130 kilometres from the finish. Later, there was a crash that took out Matteo Trentin (Mitchelton-Scott) and last year's third-placed Sebastian Langeveld (EF Education First-Drapac).
Into the Arenberg and Gilbert attacks
The previous events had turned the peloton into a rather small group of about 50 riders. They were trailing the nine leaders by 2:30 when reaching the famous Arenberg Forest. Jelle Wallays set the pace in the Trouée Arenberg, and the group broke into pieces.
The small peloton of about 50 riders was led into the forest by German champion Marcus Burghardt, working for Peter Sagan. Halfway through the 2400 metres long section Mike Teunissen (Sunweb) accelerated and only Philippe Gilbert (Quick-Step Floors) joined his move.
Once off the cobbles, only six riders survived in front, two minutes ahead of the Teunissen and Gilbert and the peloton on their heels. Nils Politt (Katusha) joined the move. Mads Pedersen (Trek) tried to close the gap too, but he punctured.
At 80 kilometres from the finish, the trio blasted by the three dropped riders from the early breakaway move. The peloton was trailing this strong group by 20 seconds. Trek moved to the front to control this breakaway move, which was neutralized ahead of pavé sector 16.
Czech champion Zdenek Stybar was next to try an acceleration, and he extended his lead on the cobbles of Warlaing à Brillon. Behind him, John Degenkolb, Lars Bak and Stijn Vandenbergh escaped the peloton.
Next up was the difficult pavé sector from Tilloy to Sars-et-Rosières. There were only four riders left in front as Paris-Nice winner Marc Soler was dropped. A strong Wallays, Dillier, Bystrom and Robeet were in front but once off this tough section, the four leaders were 40 seconds ahead of Stybar and Soler, with the peloton 20 seconds further back.
In between them, there was a group with Degenkolb, Vandenbergh, Bak and Boasson Hagen. At the feed zone, the gap between the leaders and the peloton was less than a minute, with everybody going full gas putting Ronde van Vlaanderen discovery Mads Pedersen in trouble. Stybar continued his effort on pavé sector 14 from Beuvry to Orchies while Soler sat up. Oliver Naesen and Yves Lampaert punctured out of the peloton near Orchies.
Van Avermaet lays the groundwork for Sagan
When approaching the 200km mark, only Wallays, Dillier and Bystrom remained in front. The peloton bridged back up with Stybar and Robeet while trailing the three leaders by half a minute. It seemed like the sign for Greg Van Avermaet to accelerate. His move at 55km from the finish was quickly neutralized but his second acceleration strung out the peloton.
Just like in Flanders the Belgian lacked the power to emerge alone but as the pace slowed Sagan pushed on the pedals and quickly opened a gap.
The world champion was on the break's coattails with 51km to go, while the chase from behind lacked cohesion and authority. It was telling that until that point Sagan was one of only a handful of Quick-Step's riders who hadn't been on back-foot due to crashes or mechanicals.
A few seconds, soon stretched to almost a minute as several counter-attacks were chased down. Sagan's presence at the front of the race seemed to give the break a new lease of life, before eventually Terpstra and Phinney helped to create a response that included Van Avermaet and Vanmarcke. The quartet linked up with Stuyven and Van Aert, who had attacked moments before, but as the race clicked through sectors 10, 9 and 8, the gap to Sagan only grew.
Terpstra tried to rally with 35km to go and although Bystrom cracked, and the Wallays fell back the Quick-Step rider was unable to mount a serious challenge to Sagan's growing authority.
The world champion cut a commanding figure as he led the race through the Carrefour de l'Arbre, with Diller in tow, and as the pair entered the final 10km to chase ran out of gas.
Van Aert's untimely mechanical ended his brave race, while Phinney put in one huge turn for Vanmarcke before slipping back.
Stuyven, Van Avermaet, Terpstra and Vanmarcke kept their slim hopes alive for as long as possible but up ahead the alliance between Dillier and Sagan held firm. Given what happened two years ago, when Mat Hayman shocked Tom Boonen, to win nothing was certain as the pair entered the velodrome.
Dillier's do-or-die attitude carried him to the front as he swung right and onto the track but as the line approached not even the underdog could hold back the growing inevitability.
The last 75m took place as if they were in slow motion; a chance to dwell on the cobbled classics; the Quick-Step domination, the tale of Flanders, and finally a flash of rainbow colours as Sagan closed another chapter of cycling history.
