Saadi crossed over from Libya's Saharan desert border in a convoy of vehicles and has been intercepted by local troops, Daily Telegraph quoted Niger''s Justice Minister as saying last night.
Marou Amadou, the Justice Minister, confirming the crossing over of Saadi said his convoy had reached the northern town of Agadez and was continuing onto the capital Niamey, where a number of top Gaddafi officials and several Touareg tribal leaders had sought refuge recently.
His flight may prove to be blow to the confidence of troops still remaining loyal to the old regime, who are fighting what appears to be a losing battle in Gaddafi's strongholds of Bani Walid and Sirte.
The defection of Saadi reduces the retinue of close family members still sticking by Gaddafi to just two sons, Sail al-Islam and Mutassim and his close aide and brother-in-law, Abdullah Senussi.
Saadi was one of the more westernised of Gaddafi's family. A former soccer player, he played twice for Italian Serie A teams, though he was banned for failing a drug test.
Saadi, in recent interviews had claimed that he was no longer taking part in the war and had called for a negotiated settlement.
His brothers Mohammad and Hannibal, sister Aisha and mother Safiya are in Algeria, having also fled the civil war.
Niger did not state what status Saadi would be given, but the Niger minister said the fugitive "has no status at all" in his country, indicating that he has not been refuge.