Bernard of Italy , illegitimate son of Pepin of Italy (himself a legitimate son of Charlemagne), became king of the Lombards in 810.
Edward the Martyr , briefly king of England from 975 to 978, was probably illegitimate; his father Edgar I acknowledged his younger son Æthelred as the only rightful heir
(but Edgar's opinion lost most of its strength when he died).
Vladimir
the Great became "Knyaz" of Kiev in 978, the title deriving from a Proto-Germanic word meaning "king".
He was a natural son of Sviatoslav I, a previous ruler of the Kiev Rus'.
João
I of Portugal was an illegitimate son of a previous king (Peter I) and conquered the throne after a civil war triggered by a lack of legitimate male heir.
Atahualpa was "illegitimate" son of Huayna Capac and still became Sapa Inca, roughly translated as "Emperor".
The concept of illegitimacy is not the same as in contemporary Europe; the Inca had a primary wife (formally said to be his "sister"), whose children were heirs, and many secondary wives, who could be chosen for political reasons (hence not exactly "concubines" dedicated to the ruler's concupiscence).
Atahualpa was son of one such secondary wife.
Paul
I of Russia was most probably the son of Sergei Saltykov, and not of Peter III, husband of his mother Catherine (who became Catherine II).
Paul succeeded Catherine (and was murdered 5 years later).
history.stackexchange.com/users/1261/Thomas Pornin
According to the Catholics of the time, Elizabeth I was illegitimate, since the Catholic church never recognised the marriage of Henry VIII to Anne Boleyn.
Not that Elizabeth was ever king ;-)
Even the Protestant parliament of England retroactively declared her illegitimate, with no place in the succession, when they annulled the same marriage (in 1536?).
Naturally this inconvenience was ignored when the Tudors ran out of other heirs: Edward VI and Mary I both died, and none of Henry VIII's other "official" children survived to adulthood (including his acknowledged bastard Henry Fitzroy).
history.stackexchange.com/users/4330/Steve Jessop
Cleopatra's bastard with Julius Caesar, Caesarion, ruled jointly with his mother as the last kings of the Ptolemaic dynasty of Egypt.
After Caesar's assassination, Cleopatra went on to acquire a set of bastard twins from Mark Antony.
Had they won their bid for power against Octavian, the male twin Alexander Helios would have been on track to succeed as the next Roman Emperor, but alas that story ended in tragedy.
history.stackexchange.com/users/2676/David H
Tancred of Lecce was King of Sicily.
history.stackexchange.com/users/1569/Felix Goldberg
Denmark and Norway had plenty.
Denmark had six bastard kings in a row, five of which were fathered by Sweyn II :
Harald Hen (d. 1080) Canute
the Saint (d. 1086) Oluf Hunger (d. 1095)
Eric Evergood (d. 1103) Niels (d. 1134)
Niels was followed by Erik Emune, bastard child of Eric Evergood.
Later, his bastard son Sweyn Grathe would also take the crown.
After this, there were no more bastard kings of Denmark.
The Norwegians possibly had eight bastard sons in a row (one claimed to have been such, but was not recognised), see Wikipedia .
By contrast, Sweden has had only one known bastard king, Emund the old.
history.stackexchange.com/users/13359/andejons
Manfred, Prince of Taranto, regent of Sicily 1254-1258, King of Sicily 1258-1266, was an illegitimate son of Emperor Frederick II.
Enzio, an illegitimate half brother of Manfred, married the heiress of part of Sardinia and was appointed King of Sardinia by Emperor Frederick II.
In Medieval Wales illegitimate sons who were acknowledged by their fathers had an equal right to inherit as legitimate sons.
The mothers of many Welsh kings are not known and When they are known it is often not known if they were the wives or mistresses of their fathers.
Nobody knows the proportion of legitimate and illegitimate sons among the Welsh kings.
And similarly there was a high proportion of illegitimate birth among the medieval Irish kings.
Since there were more kingdoms in medieval Ireland than in all the reset of Europe, it is possible that most medieval European kings were bastards.
King Alfonso V (1396-1458) of Aragon etc.
Left Aragon and Sicily to his legitimate brother King John II of Aragon, Sicily, etc., and his other kingdom of Sicily to his illegitimate son Ferdinand
I (1423-1494)
And I have a longer list in my anser to this question: Why couldn't bastards inherit titles?
1
history.stackexchange.com/users/20862/M.A. Golding
William the Conqueror was a bastard and I have heard him being referred to as William the Bastard by a Beefeater on a tour in the Tower of London.   Are there any other examples of bastards becoming kings in history? I'd be especially interested if any  bastards from common bloodlines ever became kings.
Which bastards became kings?
history.stackexchange.com
<royalty><royal-succession>
history.stackexchange.com/questions/12521
history.stackexchange.com/users/4495/Ronan
There are many examples of bastards becoming kings, including Bernard of Italy, many in Denmark and Norway, and potentially the majority of medieval European kings. There were many, many kings throughout European history that were bastards. It's possible in fact, that most medieval European kings were.
There were many, many kings throughout European history that were bastards. It's possible in fact, that most medieval European kings were.