Attalus III Philometor, 133 BC, leaves his kingdom to Rome.
The Wikipedia article on Attalus III has very little more to add.
Was this voluntary? There was an uprising following the occupation by Rome, with support of the neighboring kingdoms. One neighbor, Mithridates V, was a Roman ally, though he may have been playing a double game; certainly his son was doing so during his long reign.
More detail is found in the History of Rome, Vol. IV, p. 317
THE HISTORY OF ROME FROM THE FIRST PUNIC WAR TO THE DEATH OF CONSTANTINE. By B.G. NIEBUHR, published 1844. pp. 317-318
During this time Attalus Philometor, of Pergamus, had died, and with
him the dynasty of the princes of Pergamus had become extinct. The
first princes of that family were clever men, and of a mild
disposition, although much may be said against their policy, if we
take humanity as our standard. But the last Attalus was a man of
different character: his reign was tyrannical, and he himself was one
of those contemptible miscreants whom we meet with occasionally in the
history of the East, where a little natural perversity is easily
carried to the highest pitch, as in the case of the gluttony of Sultan
Ibrahim. In the East, men sometimes take a delight in what is most
unnatural and disgusting, and thus become true incarnations of a base
and satanic nature. Such a man was Attalus. The only art he occupied
himself with, was that of preparing poisons; and what amused him most
was, to get rid of those who were his nearest in kindred. He died
without issue, and left his whole kingdom to the Romans, who certainly
would not easily have recognised any one else as his successor; for
they looked upon his kingdom as their own property, which they had a
right to claim, just as a master had the right of succession to the
estate of his slave or his freedman, who died without having made a
will. The remarks of Florus, therefore on this affair are foolish. But
there was a natural son of Eumenes, the predecessor of Attalus, called
Aristonicus, who claimed the kingdom of Attalus as his lawful
inheritance; as however, there was no one in the world who could give
him assistance, it is inconceivable how he could have the madness to
believe that he would be able to hold out against the Romans, and how
it was possible for him to find any support among the people of
Pergamus. And yet the war lasted much longer than had been
anticipated. The effeminate inhabitants of the magnificent country of
Lydia and Ionia carried on the war with great resolution; and besides
them, Aristonicus had many Thracian mercenaries in his army. On the
part of the Romans, the war was badly conducted, as their generals
thought of nothing else but enriching themselves, and turning
everything into money, instead of making the proper use of their
victories: they were, in fact, glad when a powerful and wealthy town
revolted, because it afforded them an opportunity for plunder. The war
was at length brought to an end by M. Perperna and M.' Aquillius.
Aristonicus was taken prisoner at Stratonicea, and adorned the triumph
at Rome. The Romans thus acquired an extremely rich province. The end
of this, as well as of the Servile war, belongs to a later date than
the year 619, which is the year of the tribuneship of Tib. Sempronius
Gracchus. The reduction of Sicily falls in the year 620, and the
defeat of Aristonicus in 622.
