Is seeking knowledge obligatory in Islam?

Yes for what your own practice requires; beyond that it is a communal obligation.

Scholars distinguish two levels. The first is fard ‘ayn, binding on every individual: enough knowledge to perform what is already obligatory on you. Someone who prays must know how to pray; someone who trades must know which transactions are permitted. Nobody is excused from this, because you cannot fulfil an obligation you do not understand.

The second is fard kifayah, a communal obligation: the deeper study of tafsir, hadith sciences, jurisprudence and the rest. If enough people in a community take it up, the obligation is lifted from the others. If nobody does, the whole community bears the failure.

The well-known report that "seeking knowledge is an obligation upon every Muslim" is narrated by Ibn Majah. Its chain has been debated at length by hadith scholars, though many graded it acceptable on account of its supporting narrations. The underlying principle does not rest on that report alone: it follows from the plain logic that obligations presuppose the knowledge needed to carry them out.

A practical way to read this: learn first what your own daily practice depends on, then widen the circle as your situation changes.