The Nintendo DSi is a foldable dual-screen handheld game console released by Nintendo. The console launched in Japan on November 1, 2008, and worldwide beginning in April 2009. It is the third iteration of the Nintendo DS, and its primary market rival was Sony's PlayStation Portable (PSP). The fourth iteration, titled Nintendo DSi XL, is a larger model that launched in Japan on November 21, 2009, and worldwide beginning in March 2010. Development of the DSi began in late 2006, and the handheld was unveiled during an October 2008 Nintendo conference in Tokyo. Consumer demand convinced Nintendo to produce a slimmer handheld with larger screens than the DS Lite. Consequently, Nintendo removed the Game Boy Advance (GBA) cartridge slot to improve portability without sacrificing durability.

While the DSi's design is similar to that of the DS Lite, it features two digital cameras, supports internal and external content storage, and connected to an online store called the Nintendo DSi Shop. Nintendo stated that families often share DS and DS Lite consoles. Its new functionality was intended to facilitate personalization, so as to encourage each member of a household to purchase a DSi. The handheld supports exclusive physical media in addition to DS games with DSi-specific features and standard DS titles. The only exception to its backward compatibility are earlier DS games that required the GBA slot. Nintendo sold over 41 million DSi and DSi XL units combined. They were succeeded by the Nintendo 3DS.

Reviews of the Nintendo DSi were generally positive; IGN and bit-tech decried the console's lack of exclusive software and removal of the GBA cartridge slot, though its added functionality caused many journalists to recommend it to those who had not purchased a previous DS model. Numerous critics were disappointed with the limited resolution of DSi's cameras, though others such as Ars Technica and GameSpot agreed they were adequate for the handheld's display. CNET and PCWorld considered the DSi Shop to be the most important buying incentive for current DS owners. Some critics believed the DSi XL was not an essential upgrade. GamePro and Wired UK, on the other hand, praised the DSi XL's larger screens for improving the gameplay experience and revitalizing older DS games.

Development of the Nintendo DSi started at the end of 2006. It was the first time Masato Kuwahara of Nintendo's Development Engineering Department served as a hardware project leader. Work went at a quick pace to meet deadlines; his team had to devise a theme for the new DS in time for a late December presentation, and by February 2007, most specifications for a chipset had to be completed. Kuwahara reported that his team had difficulty determining the potential market for the handheld during the design process; he said of their goal, "We have to be able to sell the console on its own [without games at launch]. It also has to be able to meld into the already-existing DS market." The console's digital cameras were considered early in development: Nintendo president and Chief Executive Officer Satoru Iwata described the touchscreen as the Nintendo DS's sense of touch, and the microphone as its "ears"; a co-worker suggested that it should have "eyes". Kuwahara's team originally wanted one camera with a swivel mechanism, but this was abandoned due to concerns of reliability, cost, and the need of a thicker console. Owing to consumer demand, Nintendo also improved the handhelds' volume and audio quality and made it slimmer with larger screens compared to the Nintendo DS Lite. However, to improve portability without sacrificing durability, the GBA cartridge slot present on earlier models was removed. To compensate, Nintendo continued to support the DS Lite as long as there was consumer demand for it.

"I made the presentation [...] then at the end asked everyone if this was a game system they would want to own. The result was three to seven. Three people wanted it, seven didn't. And I imagine that since one of the designers was standing right there in front of them, some of them held back their true opinion. In truth, it was probably more like one to nine. It was as bad as I had feared."

The DSi's size was changed midway through development, delaying its release. Its original design included two DS game card slots, because of demand from both fan communities and Nintendo employees, which consequently made it larger. When the console's designs were unveiled to Nintendo Entertainment Analysis and Development producers in October 2007, it was met with lukewarm reception due to its size. However, Iwata and Kuwahara's own hopes resulted in the creation of a prototype. A quick hands-on investigation led them to abandon the dual-slot design, which made the DSi approximately 0.12 inches (3 mm) slimmer. Ever since the handheld's in-company unveiling, its internal designs were finished along with assembly and durability specifications. Iwata described having to resize the console so close to manufacturing as being essentially the same as making another handheld.

Yui Ehara, designer of the DS Lite and DSi's original casing, had to redesign the revised case. He advocated changing the six speaker apertures, as their circular perforations were redundant to the rest of the handheld's interface. He believed that this alteration also signaled a clearer distinction between the DSi and its predecessors while keeping the unit "neat" and "simple". Ehara hoped the DSi's added features would not interfere with his desired iconic image of the Nintendo DS product line: two rectangles, one on top of the other, with each half containing another rectangle inside. This model was publicly revealed at the October 2008 Nintendo Conference in Tokyo, along with its Japanese price and release date. While the DS product line's worldwide yearly sales figures consistently surpassed those of its primary market rival, Sony's PlayStation Portable, demand for it in Japan was decreasing; Nintendo's launch of the DSi was intended to stimulate sales. The company was less concerned with releasing the DSi in other territories, where DS Lite market demand remained high.

Development of a large DS Lite model in 2007 eventually led to the DSi XL. Nintendo had designed a large DS Lite model with 3.8-inch (97 mm) screens, compared to the standard 3-inch (76 mm) screens; development of this new handheld advanced far enough that it could have begun mass production. However, Iwata placed the project on hold due to consumer demand for the DS Lite and Wii. He later pitched the idea of simultaneously releasing large and small versions of the DSi instead, but Nintendo's hardware team was incapable of developing two models concurrently. After finishing work on the DSi, Kuwahara started the DSi XL project and became project leader. The DSi XL, a DSi model with 4.2-inch (110 mm) screens, was announced on October 29, 2009. Various names for it were considered, including "DSi Comfort", "DSi Executive", "DSi Premium", "DSi Living", and "DSi Deka" (Japanese for "large"). Mario creator Shigeru Miyamoto insisted on "DSi Deka". The handheld has an improved viewing angle over its predecessor, which allows onlookers to see the screen's contents more easily. This feature was absent from the large DS Lite model due to cost issues at the time, which also limited LCD screen size. The cost of LCD screens is determined by how many pieces are cut from a single large glass sheet. To keep these costs within a certain threshold, Nintendo set a screen size limit of approximately 3.8 inches (97 mm), which was later increased to 4.2 inches (110 mm).