Sasa Badnjar, a former member of Mladic's security, was arrested on Monday in Belgrade and detained for a month pending investigation as part of efforts to crack down on Mladic's network, Radio B92 reported.
Zoran Stankovic, the Serbia and Montenegro defence minister, said Badnjar was among 50 people listed in a military intelligence report as suspected of helping Mladic to hide after he was last seen in army facilities in mid-2002.
The secret report was presented to Serbian officials earlier this month and was compiled in an effort to show that the army is trying to find the fugitive and to disprove accusations that some renegade elements were still sheltering him.
Stankovic said: "The report mentions over 50 people, most of whom are army pensioners. Some were from the army security agency but they were removed last November."
The report says that when Mladic last left a military building in 2002, an army car drove him to another military facility, where he switched to another army car and was taken to an undisclosed location.
Three people were involved in this action, carried out without the knowledge of the unit's commander, Stankovic said.
Badnjar was the second suspected aide to be arrested since December, when Vladimir Vukcevic, the Serbian war crimes prosecutor, warned all helpers of the former Bosnian Serb army commander that they would be prosecuted.
Graffiti shows Carla del Ponte,
the UN war crimes prosecutor
Serbia has been under increasing Western criticism that it was not doing enough to bring the fugitive to justice and Carla del Ponte, the UN war crimes prosecutor, said in Belgrade this week she wanted to see some concrete action.
Delivering Mladic to The Hague is seen as key to Serbia's efforts to forge closer links with the European Union and Nato.
Mladic was twice indicted for genocide almost 11 years ago, for the 43-month siege of Sarajevo which claimed more than 10,000 lives and the 1995 massacre of 8000 Muslims at Srebrenica.