Google disclosed the antitrust probe in a blog post late Friday following a report by technology website SearchEngineLand.com on the investigation by the Texas authorities.
"We recognize that as Google grows, we're going to face more questions about how our business works," Google's deputy general counsel Don Harrison said.
"As Search Engine Land first reported, we've recently been approached by Texas Attorney General Greg Abbott's office, which is conducting an antitrust review of Google," Harrison said.
"We look forward to answering their questions because we're confident that Google operates in the best interests of our users," he said.
According to Search Engine Land, Abbott has been investigating since July whether Google is "manipulating its paid and editorial results in a way that violates antitrust laws."
The probe stems from complaints by three rival search companies -- Foundem, a British price comparison site; New York-based SourceTool, a website run by parent company TradeComet; and Ohio-based myTriggers.
"They claim that Google's algorithms demote their site because they are a direct competitor to our search engine," Harrison said. "The reality is that we don't discriminate against competitors."
He said that "given that not every website can be at the top of the results, or even appear on the first page of our results, it's unsurprising that some less relevant, lower quality websites will be unhappy with their ranking."
European regulators opened an informal investigation in February into similar allegations from three Web companies including Foundem.
Harrison suggested Google rival Microsoft was behind the various complaints.
He said Foundem was backed by an organization funded largely by Microsoft and that both TradeComet and myTriggers were represented by Microsoft antitrust attorneys.
The Google counsel also noted that a federal judge earlier this year dismissed a private antitrust lawsuit against Google filed by TradeComet.
Microsoft and Yahoo! teamed up last year in a bid to rival Google in search but have made only slight inroads against the Mountain View, California-based company which controls around 65 percent of the US search market.