Spurs cruised to a 3-1 victory over CSKA Moscow, which guarantees Mauricio Pochettino's side a place in the Europa League knock-out stages.
Alli produced an impressed display and was on target for the north Londoners, courtesy of a curled effort past the clutches of Igor Akinfeev.
The 20-year-old was, unsurprisingly, soon discussing his exploits with reporters in the mixed zone.
While Alli was busy analysing the effects of playing at the national stadium Dier brushed past and slapped his teammate on the back of the head before moving out of shot.
Alli initially looked perplexed and took a couple of seconds to realise who had administered the blow before laughing and muttering under his breath that Dier was "going to pay" for the interruption.
As all Spurs fans will know, the pair are often involved in tit-for-tat shenanigans.
Social media is usually the domain for the duo's banter and Alli is probably already plotting his revenge, having been caught out on camera.
Watch the video at the top of the article to see Eric Dier slap Dele Alli during his post-match interview....
