1923, as a psychological theory (in the nature vs. nurture debate), from environmental + -ism. The ecological sense is from 1972. Related: Environmentalist (n.), 1916 in the psychological sense, 1970 in the ecological sense.
word-forming element making nouns implying a practice, system, doctrine, etc., from French -isme or directly from Latin -isma, -ismus (source also of Italian, Spanish -ismo, Dutch, German -ismus), from Greek -ismos, noun ending signifying the practice or teaching of a thing, from the stem of verbs in -izein, a verb-forming element denoting the doing of the noun or adjective to which it is attached. For distinction of use, see -ity. The related Greek suffix -isma(t)- affects some forms.
Others are reading
Definitions of environmentalism
the philosophical doctrine that environment is more important than heredity in determining intellectual growth;
the activity of protecting the environment from pollution or destruction;