Sindh's capital is Karachi, a city of 18 million people that is Pakistan's economic hub, and is plagued by Islamist militant and sectarian violence as well as kidnappings and contract killings.
Provincial Information Minister Sharjeel Memon did not spell out how closing down the networks would improve security. But security services say instant messaging and Internet telephony are used by militants and other armed groups to plan attacks.
It was also not clear if Pakistan's other three provinces would follow suit, or even if the ban could be practically enforced. Officials from the Pakistan Telecommunications Authority did not return calls requesting comment.
The decision was taken after a meeting attended by the Sindh chief minister and senior police and intelligence officials.
"We are imposing a ban on WhatsApp, Skype, Viber, Tango and other networks," Memon told a news conference. "The ban will remain in place for at least three months for security reasons."
Sana Saleem, a co-founder of Pakistani Internet advocacy group Bolo Bhi, said any move to stifle Skype would be illegal.
"This is just the government trying to show they are cracking down when they have failed to control law and order problems," she said. "Sadly they are doing that by taking away people's fundamental rights."
Karachi not only suffers from frequent Taliban and sectarian bombings but also mafia-like politicians who enjoy a stranglehold on the city's security services.
Police are poorly trained, poorly equipped and most are too low-ranking to fill in a basic crime report, according to a study last year by the Asia Society.
Pakistan has taken to clamping down on Internet freedoms of late. In September 2012, the Pakistani government blocked YouTube after an anti-Islamic video was posted online.
Since then, Pakistan has blocked liberal websites although has left militant sites untouched.
Islamabad recently signed a contract with Canadian company Netsweeper, according to Canadian research group CitizenLab, after advertising for a firm that would allow it to block 50 million websites at a time.
© Thomson Reuters 2013