The 33-year-old Briton - who was jeered by fans during the fight - landed nothing of note until an overhand right ended Helenius' night.
Anthony Joshua's first stoppage win in two and half years - sets up a blockbuster fight with American Deontay Wilder in January.
"It's a fickle sport, you've got to be real about this industry and not get caught up. I've done my job tonight," Joshua told BBC Radio 5 live.
When asked in the ring about the potential fight with Wilder, Joshua joked: "My back's gone, is there a doctor in here? I want to carry this heavyweight division to the top."
Conor McGregor.
Helenius needed oxygen after the heavy knockdown, but recovered and left the ring unassisted after congratulating Joshua.
Helenius was the more aggressive fighter in the opening round, without landing any notable punches, as Joshua opted for a cautious approach against his marginally taller opponent from Finland. Joshua was more purposeful two minutes into the second round as he threw a brief flurry of punches which forced Helenius back against the ropes. They swapped respectful nods at the bell as if to thank each other for the lack of punishment in those first six minutes of wary sparring.
Joshua was due to renew hostilities with old rival Dillan Whyte on this show. But that bout had to be cancelled after Whyte returned adverse analytic findings in the pre-fight anti-doping protocols.
Deontay Wilder, Helenius was seizing an opportunity to return to action at the top tier of the heavyweight division. He began with spirit too, rushing out in the first round to cuff punches at Joshua's guard
Joshua did use his right hand sparingly in the fight’s first half and after setting it up throughout, he was finally able to execute it with significant purpose which Helenius had no answer to. 
The immediate focus for Joshua now is a showdown with Deontay Wilder next year in a mouthwatering contest that has been years in the making.
The knockout will give Joshua’s confidence a massive lift and he can now head into a Wilder battle on the back of an impressive stoppage. 
Read more
Women’s World Cup, AUSTRALIA 0-0 FRANCE Newcastle United vs. Aston Villa:5-1Sarah H
Also on site :
- GCE Global Solutions Corp. Announces Strategic Acquisition of GCE Payroll Advisers Inc. to Strengthen Global EOR and Payroll Platform
- Trump Wants To”Unleash Hell” In 2026; POTUS Tries To Go Full Maximus With A Misguided ‘Gladiator’ Flex
- Parents of Campbell Hall student killed in the school’s parking lot file wrongful death suit