Laszlo Toroczkai said that if Ukraine loses its statehood, it will claim Transcarpathia, a region home to some 150,000 ethnic Hungarians.
Toroczkai, the leader of a party with six deputies in the 199-member Hungarian Parliament, hosted a conference of European far-right leaders.
In his speech, Toroczkai made the claims and called for autonomy for ethnic Hungarians in western Ukraine.
In December, Hungarian Prime Minister Viktor Orban expressed support for Ukraine's sovereignty and territorial integrity.
The Hungarian government is reacting to Ukraine's restrictions on the rights of 150,000 Hungarians to use their mother tongue.
Hungary is the target of EU criticism
Orban, who opposes Ukraine's membership in NATO and the EU, has come under harsh criticism from EU countries for pursuing pro-Russian policies.
The EU decided last month to open full membership negotiations with Ukraine, despite Hungary's absence from the vote.