Trevor Timon, 31, hit 30-year-old Oliver Dearlove while the victim and his friends were talking to a group of women in Blackheath, south-east London, in August 2016.
Timon admitted manslaughter but was cleared of murder at the Old Bailey.
He was also given five years on extended licence as he was considered a "significant risk" to the public.
Mr Dearlove died hours after the attack, which happened after he had struck up a friendly chat with the women, who had been out celebrating a birthday with Timon.
How lives are devastated by a single blow
Timon demanded to know what they were discussing and told his victim "if you don't get out of my face I will knock you out", the court heard.
He then punched Mr Dearlove in the side of the head with his left fist, sending him to the ground and knocking him out.
A post-mortem examination found he died from bleeding to the brain.
Judge Mark Dennis QC told Timon, who had a history of violence, Mr Dearlove had presented no threat and the punch was an "unprovoked and gratuitous act".
"This was a senseless death that occurred as a result of an act borne out of a flaw in your character which in the past has seen you display unnecessary violent conduct to others when, for reasons best known to you, you have become annoyed or wound yourself up," he said.
"Oliver Dearlove was a young man of fine character with a long and promising future ahead of him. His loss in such senseless circumstances has, as the court has heard, caused untold misery for his family and those close to him."
"Many lives have been turned upside down."
Speaking before the sentence was handed down, Mr Dearlove's girlfriend Claire Wheatley broke down in tears as she described her heartbreak at losing her "Mr Right".
The nurse described him as the "most gentle, kind and relaxed person I have ever known and a true gentleman which is why so many people liked him."
She said the couple had been making plans for the future before the "dreaded knock on our door".
Miss Wheatley added: "We are all familiar with the phrase 'a broken heart', but unless you have experienced some form of loss or sadness it is difficult to understand how this truly feels.
"On August 28 2016, I was unfortunate enough to learn its true meaning.
"The difference is my heart is not broken, it feels more like it has been obliterated."
Mr Dearlove's mother Joy Wright told the court: "No parent should have to make the decision to switch off their child's life support.
"Timon may as well have used a gun or knife, the damage he did. He then just walked away casually without a thought or care for what he had done.
"I think of Oli every morning when I wake and dream of him in my sleep. I see him as a baby, a schoolboy and as the smashing young man he had become, such a gentle unassuming soul."
In a letter read to court, Timon said: "There is not a single day that I don't think of Oliver, his friends and family and the devastating effects my action has caused."
Timon was convicted of battery after he admitted punching a woman in the face, allegedly knocking her out having previously threatened to "bang" her out.
He also had previous convictions for shouting at a bus driver while brandishing a golf club and throwing a punch at a barman after being refused re-entry to a pub at closing time.
A man who killed a banker with a single punch in a "senseless" and "unprovoked" attack has been jailed for six years.