Allowed back on Twitter after a suspension, the Republican president decried the rioters' "heinous attack".
He spoke as top Democrats called for him to be removed from office, just 13 days before he is due to step down.
His statement came amid reports that an officer injured in the protests had died.
Four people have already been confirmed dead in Wednesday's mayhem, including a woman shot by a police officer.
Returning to Twitter after a 12-hour freeze of his account as the social media company said his tweets could risk more violence, Mr Trump said: "Now Congress has certified the results a new administration will be inaugurated on January 20th.
"My focus now turns to ensuring a smooth, orderly and seamless transition of power. This moment calls for healing and reconciliation."
He also praised his "wonderful supporters" and promised "our incredible journey is only just beginning".