The train operator blamed staff shortages after five of the last seven services of the day were halted.
It comes days after trains on the Edinburgh-Tweedbank line were so crowded passengers were forced to stand crammed into toilets.
Today’s problems come as ScotRail struggles to complete staff training on two new sets of trains, which were delivered late by their manufacturers and refitters.
Simon Walton, chair of the Campaign for Borders Rail, said: “This evening’s schedule on the Borders Railway has collapsed.
“Five of seven services from 5:12pm onwards are cancelled, including both late trains - the 10:12pm and 11:11pm - making the last service 9:12pm.
“There’s much comment from would-be passengers that the Borders Railway is being targeted for ’staff shortages’.
“This comes hard on the heels of crush overcrowding and assurances given to the campaign by the ScotRail Alliance that services would be more closely monitored.”
Passenger Stuart Cameron said: “Maybe ScotRail’s worst day since the Borders Railway opened in 2015.
“Seven trains scheduled tonight from Edinburgh to Tweedbank - five are cancelled.
“Simply not good enough, even by their latest poor standards.”
@DiscoGaza tweeted: “All Borders Rail services for the next four hours are cancelled due to staff shortages.
“This is an unacceptable way to run a railway.”
@BordersRailTV tweeted: “Absolutely shocking service from @ScotRail today - perhaps their darkest day on the Borders Railway since the line opened in 2015.
Of the seven trains scheduled this evening from Edinburgh to Tweedbank only two will run!
“Appalling even by their standards lately!”
On Monday, train cancellations and missing carriages caused severe overcrowding on the line.
Around 150 commuters were left stranded at Newcraighall after two peak ScotRail services were cancelled between 8am and 9am.
This was compounded by trains running with fewer than the normal number of carriages.
ScotRail said today’s cancellations were largely due to the training backlog on its new Class 385 electric trains and refurbished diesel intercity trains.
Train crew on the Borders line also work on these trains, which operate on other routes, so have to complete the training.
Industrial action by the RMT union has further hampered the process.
A ScotRail spokesperson said: “We are really sorry to our customers who are experiencing disruption on the Borders Railway this evening.
“Buses will replace trains on some services from Edinburgh Waverley.
“Customers should check @ScotRail on Twitter or download the ScotRail app for the most up to date information.
“It has been a challenging time for Scotland’s railway, but things are beginning to improve.
“More trains are arriving on time than ever before, and everyone at the ScotRail Alliance is working flat out to provide customers with the service they deserve.
“We are training more than 20 drivers and conductors every day, and this will deliver continual improvements in the coming weeks and months.”