Airing parts of bin Ladin's speech on Monday, the message condemned the 30 January elections to elect a national assembly that will draft a new constitution.
"In the balance of Islam, this constitution is infidel and therefore everyone who participates in this election will be considered an infidel," he said.
"Beware of henchmen who speak in the name of Islamic parties and groups who urge people to participate" in the election.
He also described al-Zarqawi as the "amir" of al-Qaida in Iraq and called upon Muslims there "to listen to him".
Bin Ladin added his al-Zarqawi announcement was "a great step on the path of unifying all the mujahidin in establishing the state of righteousness and ending the state of injustice".
The al-Qaida chief added that al-Zarqawi and those with him are fighting "for Allah's sake. We have been pleased that they responded to God's and his Prophet's order for unity, and we in al-Qaida welcome their unity with us", he said.
Believed to be hiding in the mountains along the Pakistan-Afghanistan border, the Saudi-born bin Ladin last sent a video message to Aljazeera before the US presidential elections.
In that statement, he for the first time clearly took responsibility for the September 11 attacks on the US and said Washington could avoid another such strike if it stopped threatening the security of Muslims.
The latest message was broadcast on the same day that two US soldiers were killed in roadside bombings. Five more were wounded in two separate blasts, according to the military.
The first attack hit a patrol in Samarra, about 100km north of the capital. The second was in Baghdad.
Suspected fighters also killed five Iraqis whose bodies were found on the roadside in the Iraqi city of Ramadi.
The attackers left a handwritten statement on the bodies saying the killed were policemen who had been pursuing mujahidin and were executed according to the sharia (Islamic law).