Hemmings scored 24 goals for Cowdenbeath last season, including two hat-tricks, and was named Scottish Championship Player of the Season.
Barnsley manager Danny Wilson said he did not believe the 22-year-old would struggle to adapt to English football.
"He has got good pace and is a very good finisher as the stats will tell you," said Wilson.
"He can play in a number of positions, but more importantly he's down the middle and has got goals in him, which is what we want to bring into the team."
Hemmings began his career at Tamworth before moving to Rangers, for whom he made 10 appearances, and then joined Cowdenbeath in the summer of 2013 after a successful loan spell.
He is Barnsley's second signing since being relegated from the Championship, following the arrival of ex-Sheffield Wednesday goalkeeper Adam Davies.

Media playback is not supported on this device
Goals from Gareth Furlong, Luke Hawker and Rupert Shipperley secured the win in Belfast.
They play Scotland on Sunday before facing France in their final group game on Tuesday,
A top-two finish in the eight-team tournament would lift them in the rankings and give them a chance of a first World Cup appearance.
The World League is a series of tournaments held over a two-year period with the latest edition running from Saturday 11 to Sunday 19 March in Belfast.
The final group standings decide the knockout fixtures and the two teams who reach the final will go through to the next round of the World League in London in June.

Its authors suggested better broadband could help create higher-paid jobs.
They have also highlighted the need for adequate provision of training and childcare to help support workers.
Highlands and Islands Enterprise, which was involved in commissioning the study, is supporting work to offer superfast broadband in rural areas.
The report, Minimum Income Standard for Remote Rural Scotland 2016, has suggested that the level of income required to afford a socially acceptable standard of living and "to participate in society" is 10 to 30% more expensive in remote rural Scotland.
However, it has also suggested falling energy costs have "to some extent" eased "the burden" on the cost of living for people living in remote rural areas.
Fergus Ewing, Secretary for the Rural Economy and Connectivity, said: "I welcome this latest Minimum Income Standard Report and note the positive impact that lower diesel and petrol prices has had on those who have to travel long distances.
"The Scottish government is already doing much to support rural communities through transport initiatives and Rural Fuel Poverty measures, and we will continue to work with rural communities to identify the best solutions to rural challenges."
The report was commissioned by HIE, Scottish Enterprise, the Rural and Islands Housing Association Forum and the Scottish Federation of Housing Associations.

In April, the authority launched a search for organisations to run the sites, which include Wales' only working windmill and a Victorian prison.
The council said budget cuts had forced it to give up the attractions.
Only one organisation has so far expressed interest.
Beaumaris courthouse and gaol, and Melin Llynnon and its ancient roundhouses, near Llanddeusant, are currently run by the council's museums and culture service.
Anglesey's head of learning, Delyth Molyneux, said: "Given the significant financial pressures faced and the fact that these sites don't form part of a statutory service, there's a real possibility that they will have to be mothballed or closed altogether if no one else can be found to run them."
Politicians had expressed concerns the courthouse and gaol could be sold to developers.
Beaumaris town councillor Jason Zalot said a sub-committee had been set up to look into running the Beaumaris gaol and courthouse.
"It's a massive undertaking for us. But bear in mind the town council have already taken over the management of the toilets.
"We have proven that we can work hand in the hand with the council."
Llynnon is currently closed and both Beaumaris courthouse and goal are open on weekends and during upcoming half term holiday.
Council leader Ieuan Williams said the council would do its best to support "interested parties" in order to ensure a smooth handover "as the success of these attractions is very important to us".
Relegated Barnsley have completed the signing of striker Kane Hemmings from Cowdenbeath on a free transfer.
[NEXT_CONCEPT]
Wales men's hockey team beat Poland in their opening game in the of the World League 2 tournament.
[NEXT_CONCEPT]
People in remote rural Scotland need up to 30% more income than the rest of the UK to have "a socially acceptable standard of living," says a new report.
[NEXT_CONCEPT]
Some of north Wales' most important heritage sites could close if no-one is found to take them over, Anglesey council has warned.