The 26-year-old Armenian striker scored either side of a Robert Lewandowski goal as Bayern Munich suffered their first league loss.
They have thus crashed back down to reality four days after their 7-2 demolition of Tottenham Hotspur in London in the UEFA Champions League.
Bayern Munich’s first home defeat in 12 months left the title race wide open with the multiple German champions remaining on top with 14 points.
They are ahead on goal difference of Freiburg, who drew 2-2 with Borussia Dortmund, and RB Leipzig and Bayer Leverkusen, who drew 1-1.
Schalke 04 conceded a stoppage time equaliser through Jonas Hector to draw 1-1 with Cologne and miss the chance to go top of the table.
Schalke 04, who ended a four-game winning streak in the league, are also on 14 points.
Borussia Moenchengladbach, on 13 points, take on Augsburg on Sunday and will go top with a win.
Hoffenheim had a golden chance after four minutes in Munich but Adamyan, who joined this season from second division Regensburg, hesitated in his one-on-one with goalkeeper Manuel Neuer.
It allowed Jerome Boateng to clear at the last moment.
Bayern Munich did not come near their midweek European form.
Apart from some good chances in the first half, they never showed the kind of clinical finishing that saw them fire seven goals past Tottenham.
Adamyan made amends in the 54th minute when he fired in from a tight angle to surprise Neuer, after Corentin Tolisso had lost possession in his own half.
Lewandowski grabbed his 11th goal of the campaign with a well-timed header after Bayern Munich upped the pressure in search of an equaliser.
Adamyan, however, crowned his first start for Hoffenheim with his second Bundesliga goal in the 79th minute.
This was after he beat Neuer once more with a low shot, to give his team their first win in Munich and their first three points since late August.
Schalke 04 thought they had won their game when Suat Serdar nodded in at the far post in the 75th minute.
But, like their Ruhr valley rivals Borussia Dortmund earlier on Saturday, they failed to hang on to their lead.
Schalke 04’s Guido Burgstaller hit the post in the 88th minute but it was Cologne who scored with Hector’s header in stoppage time for a deserved equaliser.
Hector had earlier almost scored an own goal, heading a clearance on to the post.