ACI Worldwide (NASDAQ: ACIW) is a payment systems company headquartered in Naples, Florida. ACI's products and services are designed to facilitate electronic payments and are used principally by financial institutions, retailers and electronic payment processors. ACI's portfolio of products generally addresses four primary market segments:

ACI was founded in Omaha, Nebraska as Applied Communications Inc in 1975 to develop software for communications applications making use of the new NonStop computers from the then Tandem Computers. The original founders were James Cody, Dave Willadsen, and Dennis Gates. Chuck Hackett, Charly Foglesong, and Mike May began ACI's relationship with Tandem. Many early projects involved building systems to handle ATMs, front ending the bank core banking systems, which led to the launch of the first product, BASE24, in 1982. BASE24 was originally coined by a salesman. The name was meant to represent "Baseline Software for 24 hour-per-day system operations".

Over time BASE24 was expanded to service transactions from other channels such as POS terminals and branch systems. And the product portfolio grew to cover more functions around card transactions such as clearing and settlement and fraud detection. The hardware utilised for these products was also expanded to include suppliers including IBM, HP, Stratus and Sun Microsystems.

In the 1990s ACI's product range was extended to cover wholesale banking payments with solutions for ACH transactions and wire transfers leading to the acquisition of IntraNet in 1998, which expanded the product range with the Money Transfer System product.

Since 1995, ACI has been publicly traded on NASDAQ, initially under the name Transaction Systems Architects (TSA), and since July 2007 under the name ACI Worldwide, Inc. (ACIW). Prior to that ACI had periods under US West and Tandem ownership and as a private company.

In November 2009 it was announced ACI would buy Euronet Essentis to expand card issuing and merchant management capabilities and in March 2011 agreement was reached to acquire ISD Corporation to broaden payment product offerings for retailers.

ACI purchased S1 Corporation in February 2012.

Donna Cody was one the first 10 people to work at ACI, the first woman to work at ACI and she still works at ACI (longest tenured person at ACI.) Donna works in the Software Management group and is based in Omaha. Donna was married to Jim Cody, one of the original founders.