In 854, he repulsed an attack by an army of the Bulgarian Khan Boris I and concluded a peace treaty with him, exchanging gifts.

On 4 March 852 Trpimir issued a charter in Biaći (in loco Byaci dicitur) in the Latin language, confirming Mislav's donations to the Archbishopric in Split.

[4][9] On a gable arch from an altar screen of the Rižinice monastery, carved in stone, stands a text with the duke's name and title:PRO DVCE TREPIME[RO... ...PRECE]S CHR[IST]O SV[B]MIT[TATIS ET INCLINATA HABE]TE COLA TERME[NTES...]He also likely built a church in Kapitul, in the vicinity of Knin castle, where his name is recorded from archaeological remains.

He describes Trpimir's accomplishments and his victory over a Byzantine patricius in 846, which Gottschalk connected with his theory of predestination.

[4] Trpimir was a proclaimed rex Sclavorum as a token of admiration from Gottschalk, which is also a sign of his independent rule.