Viber has released its official client for Ubuntu/Linux. To install it, open your terminal with CTRL+ALT+T and type:

wget https://download.cdn.viber.com/cdn/desktop/Linux/Viber.zip

Then unzip it and cd to that viber directory. Then do as:

unzip Viber.zip

cd Viber

~/Viber/Viber.sh

That will install Viber in your Ubuntu.

How to install in .DEB way // Currently only 64-bit available

Open your terminal and type:

wget -O viber.deb https://download.cdn.viber.com/cdn/desktop/Linux/viber.deb  

sudo dpkg -i viber.deb

That's it. You can access Viber in your Ubuntu.

Update : I came to know that some of the users are getting

home/$USERNAME/Viber/Viber: 1: /home/$USERNAME/Viber/Viber: �: not found

/home/$USERNAME/Viber/Viber: 1: /home/$USERNAME/Viber/Viber: ELF: not found

/home/$USERNAME/Viber/Viber: 1: /home/$USERNAME/Viber/Viber: Syntax error: word unexpected (expecting ")")

so that Issue can be fixed by "♦: command not found" in tty after login post.