SEATTLE (Reuters) - Adobe Systems Inc announced a $1.8 billion deal to buy business software maker Omniture Inc on Tuesday as it looks to counter falling sales from its Photoshop and Acrobat programs.

Adobe, which reported lower fiscal third-quarter sales and profit on Tuesday, has been struggling to grow in the economic downturn as corporations and home computer users rein in technology spending.

The addition of Omniture would allow Adobe to offer software to advertising agencies and corporations, allowing them to analyze how consumers use commercial websites built using Adobe tools in order to improve their profitability.

Adobe, whose software competes with products from Microsoft Corp and Apple Inc, offered to pay $21.50 per share in cash for Omniture, a 24 percent premium over Omniture's closing price on Tuesday.

Omniture shares soared 25 percent to $21.74 in after-hours trading, while Adobe shares slid 4.5 percent to $34.06.

The deal, if completed, would be Adobe's second-largest acquisition after its $3.4 billion purchase of Macromedia in December 2005.

Under the terms of the deal, Omniture will become a unit of Adobe, headed by its current chief executive, Josh James. Adobe said the deal should close in the fourth quarter of fiscal 2009 and would add to Adobe's per-share earnings in fiscal 2010.

Adobe said it would be paid a fee of $64 million by Omniture if the deal is terminated, according to a regulatory filing.

At the same time as announcing the deal, Adobe reported that fiscal third-quarter earnings, excluding items, fell to 35 cents per share from 50 cents per share a year ago. But it beat Wall Street's average forecast by a penny, according to Thomson Reuters I/B/E/S, as it continued to cut costs.

Sales fell 21 percent to $697.5 million, but beat analysts' average forecast of $686.2 million. For the fiscal fourth quarter, not counting any effect of the Omniture deal, Adobe forecast revenue and earnings, excluding items broadly in line with analysts' estimates.

(Reporting by Bill Rigby and Jim Finkle; editing Bernard Orr and Andre Grenon)