The man, aged between 65 and 75, was found fully clothed near Dove Stone Reservoir, Oldham on 12 December, 2015.
The day before, he had gone into a pub and asked the way to "the top of the mountain" after travelling by train from London to Manchester.
Toxicology reports found traces of strychnine poison but police say they still do not know who the man is.
Strychnine is now used primarily as a pesticide, particularly to kill rats.
Greater Manchester Police said no wallet, mobile phone or other identification was present when the man's body was discovered by a cyclist. Their investigation is on-going.
A spokeswoman said: "The cause of death is provisionally strychnine poisoning pending confirmation by the coroner at [the] inquest."
The man had Â£130 in cash and three train tickets, including a return to London Euston, in his pocket.
In January, police released CCTV images of the man in the hope somebody would be able to identify him.
He arrived at Ealing Broadway just after 09:00 GMT on 11 December and was next picked up by CCTV at 09:50 at Euston, buying a return ticket to Manchester Piccadilly.
After arriving in Manchester at 12:07, he spent 53 minutes wandering around the station before heading into the city centre.
The next sighting was at the Clarence pub in Greenfield, Saddleworth, where he asked the landlord how he could get to the top of the hill.
Police describe the man as white, of slim build, with a receding hairline with grey hair to the side and back.
He had a large nose that may have been previously broken, and was clean shaven.
He wore a brown heavy jacket, blue jumper, white long-sleeve shirt, blue corduroy trousers and black slip-on shoes.
A mystery man found dead on Saddleworth Moor died of "strychnine poisoning," police have confirmed.