The agency won the business, which was previously handled by Momentum, after a competitive pitch process.
Saatchi & Saatchi X will be responsible for overseeing HTC's retail marketing programme to grow its brand awareness.
The appointment follows that of Mother in July to handle HTC's pan-European advertising activity.
Since then, the company has called a review of its estimated £100 million global advertising business, as it looks to increase marketing support for its products. HTC is looking for a global creative and strategic lead agency across all markets but will retain its relationships with local shops.
Kim Stoute-Purvis, the EMEA head of retail marketing at HTC, said: "We were looking for an agency that could help us become the most talked-about, sought-after brand in EMEA."