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The index rose to 56.8 in February from last month's 55.6 reading.
Economists predicted it would fall to 54.0 after unexpectedly exploding higher last month.
Any number over 50 indicates expansion, whereas a number under 50 indicates contraction.
The gains in the index were led by new orders, which rose to 60.2 from 58.2, and order backlogs, which rose to 50.9 from 46.6.
Production moderated slightly to 60.2 from last month's 60.9 reading.
Employment, which drove last month's big gains, fell to 55.7 from 58.0.