The ₽ character is a unicode 0x20BD. You can enter this hexadecimal value from the keyboard when your keyboard input source is set to Unicode Hex Input. If necessary, see the section below titled “Adding Unicode Hex Input Method”.

If you only have two keyboard input sources, the you can enter the following to get the ₽ character. Here, I assume the keyboard input source is not already Unicode Hex Input.

control+alt+space
alt+20BD
control+alt+space

Here, control+alt+space means to hold down the control and alt keys while pressing the space key. Also, alt+20BD means to hold down the alt key while pressing the 2, 0, B and D keys.

Adding Unicode Hex Input Method

1. Goto the Keyboard pane of the System Preferences application and select the Input Sources tab.

2. Select on the + button to add a new keyboard input method.

3. Scroll down to the bottom and click on Others.

4. Highlight Unicode Hex Input, as shown below.

   

5. Select the Add button, then make sure the “Show input menu in menu bar” box is checked off, as shown above.

6. Select the Shortcuts tab, then highlight Input Sources. Make sure the “Select the previous input source” and “Select next source in Input menu” boxes are checked off, as shown below.

   

References

• How can I type unicode characters without using the mouse?
• How to Use Unicode Hex Input Method in Mac?
• How do I type a Unicode special space characters without installing the Unicode Hex Input Keyboard?
• "How can I set up a keyboard shortcut for switching input source?"

tl;dr

• In HTML: ₽ or ₽
• Alternatively display standardized 3-letter code in text: RUB
• In Java code: "₽" or Character.toString( 8_381 )

Details

Russia changed the Ruble symbol around 2013-12.

See the Wikipedia pages on Russian ruble and on Ruble sign for details.

Unicode 7: U+20BD

The Unicode consortium officially designated a code point, U+20BD RUBLE SIGN, numbered 8,381 (decimal) and U+20BD (Unicode hex), for the new symbol in Unicode 7.0, released 2014-06-16. See the last entry in this Unicode chart of Currency Symbols (PDF).

Do not confuse this new character with either the Armenian letter ք or the Latin letter Ꝑ.

By the way, Unicode 7 also gained the manat used in Azerbaijan, U+20BC MANAT SIGN, ₼. More importantly, Unicode also got the Spock hand sign, Vulcan Salute, U+1F596, .

Fonts

Older fonts will, of course, lack a glyph. You will need to obtain new or updated fonts.

Character Entity

I don't understand what a "HTML code" is in your question. If you meant a character entity reference, I do not know of any named reference. You can use ₽ in your HTML to reference by the decimal number of the Unicode code point.

Use an abbreviation: RUB

As an alternative to a symbol, you can always use the 3-letter abbreviation RUB. Standardized internationally in ISO 4217.

Use An Image

Yes, you could use an image.

If you must support older computers or older browsers not yet updated with support and glyphs for Unicode 7, then using an image or the RUB abbreviation are the only sure solutions.

Wikipedia image

That Wikipedia page provides an image drawn in SVG format under a Creative Commons license.

lenta.ru

Perhaps this large sans-serif rendering from lenta.ru might be legally available; I don't know as I don't read Russian.

http://icdn.lenta.ru/images/2013/12/11/12/20131211120143352/pic_a717232efc27b9b9fb68fea1c9cd3d72.jpg

Here is the official specification diagram of the symbol:

If you can use either XeLaTeX or LuaLaTeX and can make use of a font that provides the required symbol, you could proceed as in the following example.

\documentclass{article}
\usepackage{fontspec}
\setmainfont{PT Sans} % per Unicode, the 'ruble' symbol is in slot U+20BD
\newcommand\ruble{\char"20BD }
\begin{document}
\textyen\ruble\textdollar
\end{document}