The deficit for the year ending June 30 followed a profit of more than £11m (€14.9m) in the 2013-14 campaign.
Revenue fell by more than a fifth to £51.1m, a fall attributed to a drop in transfer income and the failure to reach the Champions League.
Celtic still made a £6.8m gain on transfers but that was down from more than £17m in the previous 12 months. Celtic sold goalkeeper Fraser Forster to Southampton in summer 2014 for £10m while bringing Craig Gordon in for free as his replacement.
Their transfer spending — for the likes of Stefan Scepovic, Stuart Armstrong, Gary Mackay-Steven and a series of loan signings — was listed as £9.4m.
Chairman Ian Bankier said: “These results, which show an operating loss of £3.6m compared to a profit of £11.8m last year, reflect two key factors. First, lower contribution from the sale of player registrations, and second, diminished income from competing in the Uefa Europa League.” Celtic have since failed to qualify for the Champions League group stages for a second season but have sold Virgil van Dijk to Southampton in a deal worth a potential £13m.
Chief executive Peter Lawwell described the season as a “transition” on the park under new manager Ronny Deila and a “challenging” one off the field: “Our decision not to transfer certain players registration during the period, together with failure to progress in the Uefa Champions League, have had a significant impact on revenues and profits.”
Meanwhile Deila insists his team’s visit to Aberdeen today (12.30pm) comes far too early in the season to be of real significance to the title race.
The Hoops won all four meetings between the clubs last term on the way to their fourth successive title – the first under the Norwegian – but Deila admits last season’s runners-up have improved.
“Last year we started talking about the title race in January. Now we start in September,” he said.
“We’re going to play a lot of matches and nothing will be finished after Saturday but the team who wins that game will put themselves in a better position and get energy and confidence for the next games.
“I think Aberdeen are a better team this year than last year because they have consistency. We go into every game to win and that’s what we want, to go to Aberdeen and get three points and that would, of course, put us in a very good position.”