The app includes Google Assistant, an artificial intelligence program which makes live suggestions as you chat.
"You no longer need to leave a conversation with friends just to grab an address, share your favorite YouTube video, or pick a dinner spot," Google said in a blog post.
"Google Allo can help you make plans, find information, and express yourself more easily in chat. And the more you use it, the more it improves over time," Google said.
The app will adjust according to whether its user's style is usually an emoji or written response, for example.
Like rival apps, it has tools for personalising chat messages including changing the size of emojis and a choice of stickers.
Allo was unveiled by Google in May at the same time as Google Duo, an app for video calls, which hit the market last month.
The app is available on smartphones using Google's Android system and Apple's iOS.
Google faces a challenge in carving a place in a sector dominated by popular apps
such as WhatsApp, which Facebook acquired for some $20 billion in 2014 and which counts more than a billion users.
In July, Facebook said that users of its own Messenger application had also passed the one billion mark.
Apple has also moved to catch up with rivals, upgrading the messaging app in its latest iOS 10 launched last week.
Google and its earlier messaging app Hangouts had failed to keep pace in a market that also includes Snapchat, Skype and global rivals such as WeChat, Viber, Line, Kik and Telegram.
Conversations in Allo will be encrypted, according to Google. And additional privacy will be offered with an "incognito" option in which messages will disappear after a fixed period.
But some analysts expressed disappointment that Google did not go further in agreeing not to store messages on its servers, where they can be accessed by authorities.
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