Videos on cryptocurrency using images of billionaire businessman Elon Musk appeared on the YouTube channel.
The Twitter feed appeared to retweet several posts related to NFTs - a type of electronic artwork for investment.
The Army confirmed the "breach", saying it took information security "extremely seriously" and was resolving the issue. Both accounts have now been restored.
An Army spokesperson added: "Whilst we have now resolved the issue an investigation is ongoing and it would be inappropriate to comment further."
It is not clear who is behind the hacking incidents, which also saw the accounts renamed.
At one stage, the Twitter account name was changed to Bapesclan, accompanied by a profile picture featuring an ape-like cartoon figure with make-up mimicking a clown.
By Sunday evening, the account had been restored to normal.
The Army later tweeted: "Apologies for the temporary interruption to our feed. We will conduct a full investigation and learn from this incident. Thanks for following us and normal service will now resume."
Conservative MP Tobias Ellwood, chair of the Commons defence select committee, said what had happened "looks serious".
"I hope the results of the investigation and actions taken will be shared appropriately," he added in a tweet.
It is not the first time that a high-profile account has been targeted on Twitter.
In July 2020 major US accounts were taken over by hackers in an apparent Bitcoin scam.
Those accounts affected included Musk, Jeff Bezos, Bill Gates, Barack Obama, Joe Biden and Kanye West.