(Reuters) - Adobe Systems Inc plans to pay $1.8 billion for fast-growing business software maker Omniture Inc as the maker of Photoshop and Acrobat looks to turn around declining sales.

Adobe, which announced the deal on Tuesday as it reported lower quarterly sales and profit, has been struggling over the past year as the recession hurt technology spending and customers declined to upgrade older versions of its programs.

The acquisition would give Adobe a new stream of revenue to offset that decline. Omniture charges customers fees based on monthly website traffic, so sales are less sensitive to economic swings than Adobe.

"There is no way Adobe can grow organically. This is a smart move," said Global Equities Research analyst Trip Chowdhry.

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Advertising agencies and companies use Omniture's software to analyze how consumers use websites. It is the biggest provider of such services, competing with Google Inc and other smaller players. The vast majority of all professional websites are built with Adobe's Creative Suite line of design software.

Janney Montgomery Scott analyst Sasa Zorovic said Adobe's customers will not necessarily choose to subscribe to Omniture's services simply because its technology is embedded into Creative Suite.

"It will require some selling, but I think the opportunity is there," he said.

Adobe, whose software competes with products from Microsoft Corp and Apple Inc, agreed 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 would be Adobe's second-largest acquisition after its $3.4 billion purchase of Macromedia in December 2005.

Omniture would 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.

Adobe also reported on Tuesday that fiscal third-quarter earnings, excluding items, fell to 35 cents per share from 50 cents per share a year ago. That beat Wall Street's average forecast by a penny, according to Thomson Reuters I/B/E/S.

Second-quarter 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)