Has any country won a war, but lost a non-trivial portion of its territory?

Would British Empire loosing most of its colonies within 10 years after winning WWII count? Although the loss was not immediate, it was triggered by the loss of geopolitical positions due to expenses of WWII and rise of Britain's WWII allies.

How about all the little German principalities that were on the winning side of the Wars of the Umpteenth Coalition from 1792-1815 but were wiped off the map nonetheless?

Also, considering that same conflict, Austria was ultimately victorious, but lost Belgium.

Arguably Pontiac's War. Not sure how you measure who won/lost that war; you could look at the treaty and at the changes forced on the British. But there is no doubt that the indigenous Americans lost territory and much much more.

Czechoslovakia won WWII (or at least was on the winning side) and lost Subcarpathia to the Soviet Union. Unlike Poland, the Czechoslovak government was not a Soviet puppet, the annexation (ehm, voluntary expression of the workers' desire to join their brothers in Ukrainian SSR) was unexpected and unwelcome (though the territory was not worth much and Czechoslovakia was probably in better shape economically without it), and there was no compensation.

Serbia was on the allies side in World War 1 but it didn't exist after it and became Yugoslavia instead. Even though they expanded their land it was no longer Serbia so it technically lost 100% of its land.

India had won some territory in the 1971 Indo-Pakistani war, which was a victory for India and newly independent Bangladesh. However, India ceded the territory in the 1972 Simla agreement as a gesture of goodwill.

Ethiopia was militarily successful in the Italo-Ethiopian War of 1887–1889, but still lost territory.

... while Ethiopia had been successful in the field, the Italians had managed to occupy territory and retreat in an orderly way. They retained their acquisitions on the Red Sea. [Ethiopian emperor] Menelik recognized the Italian occupation of his rivals' lands of Bogos, Hamasien, Akkele Guzay and Serae in exchange for guarantees of financial assistance and continuing access to European arms and ammunition.

Israel captured the Sinai Peninsula in the Six-Day War of 1967, yet despite winning another war would give it up at the negotiating table a decade later.

At the time of cease-fire Israel was winning the Yom Kippur War of 1973, having reversed their early losses, encircled the Egyptian 3rd Army, crossed the Suez Canal, threatening the regional capital of Ismailia, and advancing on the capital Cairo.

Yet in 1978 negotiating the peace treaty at Camp David, Israel voluntarily gave up the Sinai Peninsula at the negotiating table in exchange for demilitarization and UN monitoring of the Sinai. Israel secured their western border, and Egypt got their land back.

War between Chile and the coalition of Peru and Bolivia During this war, Chile payed with territory (Patagonia territories) to Argentina in order to prevent Argentinian intervention in the war against Chile. Thanks to this deal, Chile won the war and several territories from Peru and Bolivia. But Chile lost any demand over territories on the Patagonia east of Los Andes mountains, territories twice the size of Germany.

Despite being outnumbered and outgunned Finland managed to repel a Soviet invasion in the Winter War of 1939-40 although they lost 11% of their land area.

Such scenario is not uncommon during the decolonization-related wars of the 20th century. The colonial power is often able to defeat the other side militarily, but for political or other reasons had to withdraw. Some examples:

• Indonesian National Revolution (1945-1949), was considered to be a Dutch military victory, since Dutch military forces were able to defeat the Indonesian republicans and maintain control of Indonesia's major towns, cities and industrial assets. However, diplomatic pressure as well as the continuing guerrilla warfare forced it to accept the independence of Indonesia, formerly its colony since the 17th century.
• Suez Crisis (1956). Egypt nationalized/seized the Suez Canal (which was previously held by the British) in 1956. Britain, France and Israel subsequently attacked and defeated Egypt, but political pressure forced them to withdraw and accept Egypt's control of the canal.

I have read your question as "Has a country ever won a war, and still "lost" territory, and if so, why?"

One example was the Austro-Sardinian War, otherwise known as the Second War of Italian Independence.

Sardinia (Piedmont) won the war. She did so with the help of France, under Napoleon III. She had to cede her holdings of Savoy and Nice to France to obtain this help, (thereby "losing" territory to France). She was "compensated" by getting Lombardy from Austria in a winning war. Shortly thereafter, Piedmont's enhanced military and diplomatic stature enabled her to annex other parts of northern and central Italy, and ultimately unite all of Italy under her rule.

Some examples from WW2:

Poland was on the winning side but lost the eastern half of the country to the USSR. Yes they got compensated by German territories, but we could have some fun debating the relevance of that.

Britain came out a winner but it's impoverishment was a significant factor leading to withdrawal from the Empire.