Who was the first European to visit China?

Carpini, Giovanni de Piano (circa 1180-1252), Italian Franciscan friar, historian, and the first European to record a visit to China:

https://en.wikipedia.org/wiki/Giovanni_da_Pian_del_Carpine

"In 1260 two Venetian merchants arrived at Sudak, the Crimean port. The brothers Maffeo and Niccilo Polo went on to Surai, on the Volga river, where they traded for a year. Shortly after a civil war broke out between Barka and his cousin Hulagu, which made it impossible for the Polos to return with the same route as they came. They therefore decide to make a wide detour to the east to avoid the war and found themselves stranded for 3 years at Bukhara.

The marooned Polo brothers were abruptly rescued in Bukhara by the arrival of a VIP emissary from Hulagu Khan in the West. The Mongol ambassador persuaded the brothers that Great Khan would be delighted to meet them for he had never seen any Latin and very much wanted to meet one. So they journeyed eastward. They left Bukhara, Samarkand, Kashgar, then came the murderous obstacle of the Gobi desert. Through the northern route they reached Turfan and Hami, then headed south-east to Dunhuang. Along the Hexi Corridor, they finally reached the new capital of the Great Khan, Bejing in 1266. ... Although the Polo brothers blazed a trail of their own on their first journey to the East, they were not the first Europeans to visit the Mongols on their home ground. Before them Giovanni di Piano Carpini in 1245 and Guillaume de Rubrouck in 1253 had made the dangerously journey to Karakorum and returned safely; however the Polos traveled farther than Carpini and Rubrouck and reached China."

So this source credits his father and uncle with actually reaching Beijing and being the first to get all the way to China. Marco accompanied them on their second trip. The extent of his travelling and his prolific writing about it have made him the name we remember.

http://www.silk-road.com/artl/marcopolo.shtml

I suspect that this is one of those questions that is likely unanswerable, as it depends on who exactly counts as a "European" and "visit".

For example, it seems that the Romans were in China around 166 A.D.

"The first group of people claiming to be an ambassadorial mission of Romans to China was recorded in 166 A.D. by the Hou Hanshu. The embassy came to Emperor Huan of Han China from "Andun" (Chinese: ??; Emperor Antoninus Pius), "king of Daqin" "

https://en.m.wikipedia.org/wiki/Sino-Roman_relations#First_Roman_embassy
http://www.silk-road.com/artl/romanenvoy.shtml

It's quite possible that earlier people originating on the European continent visited China.

Possibly the people now known as the Tarim mummies dating from 1800 BCE to as long as 5,000 years ago.

https://en.m.wikipedia.org/wiki/Tarim_mummies

The Tarim mummies are a series of mummies discovered in the Tarim Basin in present-day Xinjiang, China, which date from 1800 BCE to the first centuries BCE. The mummies, particularly the early ones, are frequently associated with the presence of the Indo-European Tocharian languages in the Tarim Basin, although the evidence is not totally conclusive and many centuries separate these mummies from the first attestation of the Tocharian languages in writing. Victor H. Mair's team concluded that the mummies are Europoid, likely speakers of Indo-European languages.