Pegasystems Inc. on Wednesday posted third-quarter profit and revenue that fell short of expectations, and the business-software company gave a disappointing forecast of full-year results.

Its shares plunged 17 percent in after-hours trading after the report.

Pegasystems said net income for all of 2011 will be about 15 cents per share, or 61 cents per share excluding special items. Analysts were expecting adjusted earnings of $1.17 per share.

The company said adjusted full-year revenue would be about $410 million — analysts expected $442.7 million — and more than $500 million in 2012, when analysts were looking for $521.1 million.

The Cambridge company said third-quarter software license signings fell compared with a year ago, but new business was up sharply despite the weak economy. The company said more customers were choosing term licenses rather than perpetual ones, which it said would tend to push revenue into future quarters.

Net income rose to $5 million, or 13 cents per share, compared with $3.1 million, or 8 cents per share, a year earlier. Adjusted income, excluding items such as amortization and stock-based compensation costs, was 15 cents per share.

Revenue grew 6 percent to $95.5 million.

Analysts expected adjusted earnings of 31 cents per share on revenue of $111.1 million, according to a FactSet survey.

The shares fell $3.53, or 8.7 percent, to $37.04 in regular trading before the results. In extended trading, they plummeted another $6.13 to $30.91.