The last members of the group crossed the border after the Thai army evicted them from a refugee camp the previous day.
The UNHCR said the army had deported the Hmong from two camps, one in the northern province of Petchabun and another in the village of Nong Khai in Thailand's northeast.
"UNHCR was given no access to people in the first camp, while those in Nong Khai were all recognised refugees," the agency said in a statement.
On Monday, Thailand also sent back a separate group of 158 Hmong with recognised UN refugee status, in a move the UNHCR said was a breach of international law.
A Lao government spokesman said the concerns were "groundless" and the Hmong being repatriated were illegal migrants who would be housed in resettlement villages.
Thailand and Laos reached an agreement in March to repatriate the Hmong.
Ban Ki-moon, the UN secretary-general has also voiced concern, saying in a statement that he regretted "that these deportations have taken place in the face of appeals from the United Nations High Commissioner for Refugees (UNHCR) and despite the availability of third country resettlement solutions for those recognised as refugees".
|The UN says some Hmong have refugee status and need international protection [AFP]
Ban also called on the Thai and Laotian government "to take to take all necessary steps to respect the rights of those concerned and to facilitate humane solutions".
The Hmong had sought asylum after their alliance with the US during the Vietnam War, fearing they could be persecuted by the Lao communist government.
Known as America's "forgotten allies", the ethnic group from the remote mountains in Laos were recruited by the US Central Intelligence Agency (CIA) to fight alongside US forces.
More than 300,000 Laotians, mostly Hmong, fled to Thailand citing political persecution after the Pathet Lao communists took power in 1975.
Most were resettled in third countries with many now living in the US.