I’m pretty sure Lord Sakka became an Ariya after the Buddha answered one of his great questions. Also, non-returner devas attain Arahantship.
So yes, devas can become ariyas and attain enlightenment.
Later posters seem to be responding about whether a deva can attain enlightenment, in the abstract. But the question was also about becoming a buddha, i.e. like Gautama Buddha, or an ancient buddha like Dipamkara Buddha. That may be where the confusion is coming in.
In the DA / DN, this is down to a formula, spelled out clearly in DN 14, and includes the bodhisattva living a previous life in Tusita Heaven, before taking rebirth for the last time as a human:
When Vipassī, the being intent on awakening, passed away from the host of Joyful Gods, he was conceived in his mother’s womb, mindful and aware. This is normal in such a case.
It’s normal that, when the being intent on awakening passes away from the host of Joyful Gods, he is conceived in his mother’s womb. And then—in this world with its gods, Māras and Brahmās, this population with its ascetics and brahmins, gods and humans—an immeasurable, magnificent light appears, surpassing the glory of the gods. Even in the boundless desolation of interstellar space—so utterly dark that even the light of the moon and the sun, so mighty and powerful, makes no impression—an immeasurable, magnificent light appears, surpassing the glory of the gods. And the sentient beings reborn there recognize each other by that light: ‘So, it seems other sentient beings have been reborn here!’ And this galaxy shakes and rocks and trembles. And an immeasurable, magnificent light appears in the world, surpassing the glory of the gods. This is normal in such a case.
The presumption shown in DN 14 is that buddhahood is attained as a human being, and is an event of cosmic importance. DN 26 sets forward the next buddha as Metteyya:
And the Blessed One named Metteyya will arise in the world—perfected, a fully awakened Buddha, accomplished in knowledge and conduct, holy, knower of the world, supreme guide for those who wish to train, teacher of gods and humans, awakened, blessed—just as I have arisen today.
Deity is simply the English cognate to deva:
deity (n.)
c. 1300, deite, “divine nature, godhood, attributes of a god;” late 14c., “a god, God, the Supreme Being or self-existing spirit,” from Old French deité, from Late Latin deitatem (nominative deitas) “divine nature,” coined by Augustine from Latin deus “god,” *from PIE deiwos"god," from root *dyeu- “to shine,” in derivatives “sky, heaven, god.” From 1580s as “a being to whom a divine or godlike nature is attributed.”
…
*dyeu-
Proto-Indo-European root meaning “to shine,” in derivatives “sky, heaven, god.”
It forms all or part of: adieu; adios; adjourn; Asmodeus; circadian; deific; deify; deism; deity; deodand; deus ex machina; deva; dial; diary; Diana; Dianthus; diet (n.2) “assembly;” Dioscuri; Dis; dismal; diurnal; diva; Dives; divine; joss; journal; journalist; journey; Jove; jovial; Julia; Julius; July; Jupiter; meridian; Midi; per diem; psychedelic; quotidian; sojourn; Tuesday; Zeus.
It is the hypothetical source of/evidence for its existence is provided by: Sanskrit deva “god” (literally “shining one”); diva “by day;” Avestan dava- “spirit, demon;” Greek delos “clear;” Latin dies “day,” deus “god;” Welsh diw, Breton deiz “day;” Armenian tiw “day;” Lithuanian dievas “god,” diena “day;” Old Church Slavonic dini, Polish dzień, Russian den “day;” Old Norse tivar “gods;” Old English Tig, genitive Tiwes, name of a god.
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I base my answer on various talks by Ajahn Punnadhammo (specialist on buddhist comsology and various realms of existence), some talks by Ajahn Brahmali and of course overall buddhist suttas.
Long story shory, devas can mean both “Spirits” and “Gods”. There are various forms of devas, and there are at least 6 levels of devic existence.
Some devas are just minor spirits… but there is importaint distinction between deva minor-spirit and preta ghost. Preta is a ghost whos existence is mostly very miserable and it is considered very unfortunate rebirth, only hellish birth is a worse one. Some “ghostlike” spirits like dryads, elves of whatever can be considered devas, because even if their power or existence is not “that godly” like higher devas, they’re spiritual beings that life is not very miserable.
Devas of higher realms are deities that posses much better bodies than humans and you could start considering them godly in terms of their spiritual power, lifespans and general bliss and way of their existence. The higher the devas the more pure and spiritual they get, to the point of getting more and more similar to brahmic beings, that are completely beyond sensuality.
Compare it to animal realm… both an mosquito, fly, a dog and cat is an “animal” in buddhist terms, even thou most people would say that dog has much higher consciosuness and general more fortunate rebirth than a fly. But it is still animal realm. And in the same way, minor-spirit and powerful deity are both devas, even thou they are completely different levels of devas. Still even lowest devas are generally considered as capable as humans spiritually, but having more pleasurable life overally. Does having more than less pleasurable life is better to attain awakening is completely another thing…
But importaint is that any being that is capable of understanding and mindfulness can get enlightened. So if devas know teachings of the Buddha, they can practice and realise Nibbana, whatever level of devas they are.
Imagine world of regular devas (deities) just like our world but much more happy. On the one hand it is easier to practice because their mind power is surely higher than regular human being. But on the other hand, temptation of being distracted by wordly sensual objects is even higher, because devas pleasures are much more pleasurable than those of humans, and because they don’t suffer illness (at least not as much as humans).
So… imagine being a Bhikkhu in a deva realm. On the one hand it would probably be easier to get jhanas due to higher development of consciousness that they worked for in previous lives, but imagine the temptation of getting out of ascetisism and just get into heavenly sensual pleasures.
So I guess it is just like here. Most of humans don’t practice, and very few get enlightened, even if sasana is present in the world.
As Buddha had tought Gods as well, they probably have some sasana as well, at least in some of their worlds, whether they are purely spiritual or somewhere in the cosmos. But how much they practice is another thing…
Anaway, the the “deva” in buddhism can mean all these things: minor-spirit, godlike deity with amazing physical or semi-physical body, godlike deity with almost purely spiritual body like brahmas. There are many interpretations… some say that some devas live on Earth but can be only seen by divine eye. Some say that some devas can live in physical form on another planets. And clearly higher devas transcent matter and live in purely spiritual realms that could be reached only through psychic powers. All of these can co-exist and all would be considered “devas” in buddhist terms.
So to give more or less definition of deva: it is a being that can live on continuum of both spiritual or physical, that is capable of understanding and mindfulness and overall higher psychological/spiritual functions, that can influence its kamma (does not acts purely instinctively like lower beings), and is considered that avaragely have less suffering and more pleasure in existence than humans, but are not as purely spiritual as brahmas.
Who is interested in this matters, I highly recommend these entire series with Ajahn Punnadhammo and Ajahn Sona on buddhist cosmology:
Also somewhere in this playlist, in one of the talks Ajahn Brahmali spoke about devas and that they clearly can get enlightened just as we do. But there are 30 talks and I don’t know which one. Anyway all of them are absolutely great 
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