George Osborne confirmed that VAT on the charges would be scrapped when the two bridges return to public ownership.
The current charge is £6.50 for cars and £13.10 for vans, which will also pay the £5.40 rate from 2018.
But the Freight Transport Association (FTA) said the reduction for lorries and buses - from £19.60 to £16.30 - was a "kick in the teeth" for its members.
Severn Bridge tolls are charged on vehicles crossing from England into Wales.
FTA spokesman Ian Gallagher warned that "by 2018 we anticipate that the toll will be in excess of £20 for HGVs".
However, Keith Palmer from the South Wales Chamber of Commerce welcomed the chancellor's move, saying the tolls had "long been a stumbling block for the Welsh economy".
"The Severn tolls are widely regarded as a tax on Wales' competitiveness and a tax on businesses," he said.
"We welcome the Chancellor's decision to reduce tolls on the Severn Bridge from 2018 so that Wales has the same opportunities to attract inward investment as other parts of the UK."
Liberal Democrat Cardiff Central MP Jenny Willott said the move was thanks to Lib Dem ministers "pushing hard" for it, promising her party would scrap the tolls completely if it is in power after May's general election.
In his Budget speech, the chancellor also confirmed negotiations had started on plans to build a £1bn tidal lagoon in Swansea Bay, and on a city deal for Cardiff.
Severn Bridge tolls for cars and vans will be cut to £5.40 in 2018, the chancellor said in his Budget speech.