The gnomes, known as "Coolmen", are the property of the left wing Social Democrat Party.
They were being used as political campaign advertisements in the run up to provincial elections in Vorarlberg on 21 September.
Reinhold Einwallner from the Social Democrats told the BBC there were suspicions that the gnomes may have been removed by rival party the OeVP.
Gnome facts
Source: Garden historian Twigs Way
Mr Einwallner said the 400 gnomes in question had been attached to lamp-posts in around the towns of Rankweil and Bregenz.  But on Saturday night, they disappeared.
"An interesting detail is that now OeVP [the conservative People's Party] campaign posters are now hanging at the same height on many of these lampposts," he said.
Mr Einwallner said the Social Democrats were happy if individual members of the public wanted to take a Coolman gnome for their gardens but he said the party could not just stand by when campaign materials worth 3,000 euros ($3,958; Â£2,391) simply disappeared.
It would be "a major scandal" if the gnomes had been stolen by another political party, he said in a statement.
The Social Democrats have filed an official complaint with the police.
A reward of 1,000 euros has been offered to anyone with information about the gnomes' whereabouts.
In using gnomes for its election campaign, the Social Democrats are breaking with long established custom in Austria. Traditionally posters are used.
The Social Democrats have ordered 20,000 gnomes, which they plan to roll out over the next few weeks of campaigning.
Four hundred garden gnomes have gone missing in Vorarlberg in west Austria.