Christian Morley, 39, was set upon as he pursued two men he had spotted tampering with a machine he had chained up in the front garden of his home in Victoria Road East, Hebburn.
As they fled after being disturbed at around 2.20am, a third suspect he did not know was present silently approached and cut open his left arm.
Mr Morley stumbled home bloodied and was rushed to South Tyneside District Hospital by ambulance, as police sealed off the scene.
His mum, Susan Morley, 65, said thieves had stolen another motorbike belonging to her son in a copycat raid last week.
A replacement bike was also targeted by yobs at around 11pm on Wednesday.
On that occasion Mr Morley was alerted by noise outside his living room window and also gave chase, causing the thieves to flee empty handed.
But Mrs Morley, who lives with her son, believes the same gang returned in the early hours of today to finish off the job.
The widow said: “These people must be caught – they cannot go around trying to steal bikes and then using a machete as a weapon.
“I had fallen asleep on the settee and awoke to see Christian hanging onto the front door, covered in blood.
“He said he had run after two men and had been slashed from behind by a third person who he did not know was there.
“He chased them but they turned on him. He thought there was only two of them present.
“The cut to his arm was very deep, right down to the muscle, and there was lots of blood. We have been told he needs surgery today.
“His bike was stolen last week and he had been on edge ever since, which was made worse when thieves tried to take his new bike two nights ago.
“Last night he saw a couple of lads acting suspiciously across the road and told me he thought it might be the same people.
“In a way he got lucky with his attack, it’s bad enough but it could have been so much worse.
“What makes these incidents so much worse is that Christian doesn’t bother anyone, he just goes to work and comes home.”
Grandmother Mrs Morley believes the same thieves were responsible for all three incidents.
She also fears they live locally and had spent time checking out her home.
Mr Morley, a warehouse worker with Asda, told her two men wearing dark clothing and blacked-out motorbike helmets were responsible for Wednesday’s attempted theft.
He gave the same description for those who tried to steal his bike today.
In the first and third incidents they arrived on a single moped – once coloured black, and another white .
And Mr Morley has said a white van was also present during last week’s theft and today’s attempted theft.
In the first incident, thieves cut through three security locks and were seen on CCTV loading the Â£1,600 bike into a white van.
A Northumbria Police spokesman said: "At around 2.19 this morning officers received a report a man had been attacked by a group of unknown offenders on motorbikes.
"Emergency services attended and the 39-year-old victim was taken to hospital with serious but no- life threatening injuries.
“No one has been arrested and inquiries into this incident are ongoing."
Anyone who has information about the incident should contact police on 101 quoting log 69 121018.