The right-back played in a behind-closed-doors friendly for the Lions on Tuesday afternoon after joining the Den outfit for training on Monday.
Hoyte, who signed for Middlesbrough in a £3million deal from Arsenal in the summer of 2008, has made just four appearances for the Riverside Stadium outfit this season.
The 28-year-old started the Capital One Cup defeat by Accrington in August and the 1-1 draw with Sheffield Wednesday in the Championship later the same month, but has made just two brief substitute appearances since then.
Millwall boss Steve Lomas has now turned to Hoyte to increase his defensive options and the new signing will be hoping to feature in Saturday's clash with Bolton at the Reebok Stadium.
Lomas' men have shipped a worrying 28 goals in their 14 Championship games to date and are mired in a five-game run without a win, which leaves them just three points above the relegation zone.