Police said there is no specific intelligence suggesting a threat to the event in Glasgow and the move is standard practice given the current security level of “severe”.
They urged people not to bring bags larger than an A3 sheet of paper and warned anyone with a bag should expect to be searched.
Radiohead, Kasabian, Biffy Clyro and The Kooks are among the bands on the line-up for the festival on Glasgow Green.
Staged by DF Concerts, the non-camping event was created after it was announced T in the Park will “take a break” this year as organisers look to resolve issues around its recent move to a new site in Perthshire.
Police Scotland said there will be extra security at TRNSMT, which runs from Friday to Sunday.
Superintendent Stewart Carle, the event commander, said: “Glasgow will see an influx of tens of thousands of people attending this three-day music festival - travel routes and the city centre are going to be busier than normal, so please plan your journey ahead.
“Our preparations with our partners have been ongoing for many months and the public’s safety is our top priority.
“There will be an increased security presence at the event and in the city centre. Firearms officers will be present near the venue, however it is important to stress there is no specific intelligence to suggest there is a threat to the festival - this deployment is standard in crowded places given the current security level of ‘severe’.
“Our advice remains the same - be alert but not alarmed and enjoy the festival.
“We will have plain clothed police officers on duty in and around Glasgow Green to monitor crowd behaviour and look for any suspicious or criminal behaviour. This is supported by the public space CCTV system operated by our partner Community Safety Glasgow.”
Police said there will be a zero-tolerance approach to drug taking and anyone found with illegal substances will be arrested.
Anyone who is drunk will not be allowed into the festival and those who engage in disorderly and drunken, loutish behaviour inside the venue can expect to be ejected or arrested, police warned.
Flares, smoke devices and other pyrotechnics are also banned and anyone trying to bring them in will not be allowed entry.
Mr Carle urged people to behave sensibly and plan ahead to ensure they enjoy the event.
He said: “The festival organisers have put in place medical facilities and a welfare tent for those who may become unwell, but the best advice is to drink sensibly and take plenty of water, of which there will be free supplies.”