"Since that first simple tweet over eight years ago, hundreds of billions of Tweets have captured everyday human experiences and major historical events," Twitter said.
"Our long-standing goal has been to let people search through every Tweet ever published."
Examples of when the new capability may come in handy included revisiting sports or television show seasons, learning what was revealed at conferences such as #TEDGlobal, and tracking down news about major events such as #JapanEarthquake or #ScotlandDecides.
The enhanced search tool will be rolled out during the next few days, according to San Francisco-based Twitter.
Complete search results from the entire index will be found under an "All" tab on Twitter pages for Web browsers as well as in applications for mobile devices powered by Apple or Android software.
The full index was described by Twitter as a major infrastructure investment.
Standard & Poor's gave Twitter's recent $1.8 billion debt issue a "speculative" rating of BB- last week, weighing the company's push for acquisitions against likely slow growth in earnings.
S&P said the rating, roughly in the middle of the speculative or "junk" range, acknowledges expectations of healthy user and income growth at the popular online micro-messaging and social networking service.
"The company is investing very aggressively in growth. Depending on the level of business reinvestment, Twitter may not generate positive discretionary cash flow until 2016," S&P said.
Created in 2006, Twitter has grown rapidly and went public in September 2013.
The company recently said it has 264 million regular users and another 500 million who do not sign in to visit the site.
Twitter recently unveiled new changes and services to enhance its attractiveness to users and the advertisers who pursue them.