Jamie Cox attacked Viktoria Gribovskaja at her home in Lambeth, south London, in July last year.
The Swindon-based WBO European super-middleweight title holder, who was overheard threatening to "knock out" Ms Gribovskaja, had denied assault.
But he was found guilty after a trial at Croydon Magistrates' Court.
The victim was awoken in the early hours of the morning by Cox forcing his way into her flat, the court heard.
She called 999 before he broke through her bedroom door.
The 29-year-old boxer, who appeared in court under his real name Jamie Russan, was convicted of assault by beating, violence to secure entry, harassment and criminal damage.
Alex Slater, prosecuting, said: "Viktoria said she had never been so scared in her life. She thought he was going to kill her."
He was heard in a recording of a 999 call, which lasted 21 minutes, to say: "I will knock you spark out."
At his trial last month, Cox said he had "barged" through Ms Gribovskaja's front and bedroom doors because he feared she "could kill herself". He denied putting his hands on her.
But magistrates found the victim's account of the confrontation "credible and convincing".
Nathaniel Ikeazor, representing Cox, told the court the boxer regretted the incident.
He said: "Jamie is remorseful as to how his actions affected Viktoria and affected the relationship thereafter."
Cox was sentenced to 26 weeks in prison for the assault and harassment charges, 20 weeks for violence to secure entry and 12 weeks for criminal damage, to run concurrently.
He was ordered to pay Â£1,105 in compensation to his victim and court costs, and given a restraining order barring him from contacting her indefinitely.
Cox showed no emotion as the sentence was handed down.
Speaking outside court, his trainer of 10 years, John Costello, said his legal team will appeal against the decision and are "confident he will clear his name".
Cox will be suspended from boxing for the duration of his sentence, Mr Costello confirmed.
A champion boxer who forced his way into his ex-girlfriend's flat and grabbed her around the throat has been jailed.