The portrait is attributed to Barend Graat (and dated to 1666). It was exhibited at a prestigious Dutch art fair (TEFAF in Maastricht) by the dealer A. Vecht. The dealer, Dutch media (see here NIW) and blogs have all discerned Spinoza in this portrait. (HT Wim Klever on facebook.) This is no surprise because the portrait has an uncanny resemblance to the iconic portrait of Spinoza found on the old 1000 Guilders note (beloved by drug dealers and tax evaders):
The portrait on the 1000 Gulden note was almost certainly inspired by the famous (posthumous) 'Wolfenbütteler- portrait (HT Stan Verdult):
It is not impossible that the 'Wolfenbütteler portrait' was inspired by the painting now attributed to Barend Graat. And I am inclined to believe that the same person is portrayed in the 'Wolfenbütteler portrait,' the Graat-Spinoza, and 1000 Gulden note.
There is another posthumous portrait of Spinoza (dating from ca 1680) which was attached to some editions of his posthumous works, presumably by people who knew Spinoza. In my subjective view this is not obviously the same portrayed person as Wolfenbütteler portrait'/'Graat-Spinoza,' but decide for yourself thanks to a nicely edited image by Stan Verdult:
However, it is reported that in the judgment of the Het Nationaal Forensisch Onderzoeksbureau (NFO), it is more probable that these are all the same person than not. (One wonders if they think the probabilities are the same for each of the three portraits.) I have contacted the Institute, and when I hear back from them I'll report on my findings.
Be that as it may, the 'Graat Spinoza' has the advantage of being produced in Spinoza's life-time, and for all I know we will find documentary evidence (a sales contract, an estate catalogue, etc.) linking the portrait more directly to Spinoza. But for now it is worth being skeptical, and I would like to offer some reasons for this.
First, as Julie Cooper has argued Spinoza was against intellectual celebrities and cults of personality, which he identified with Descartes. This is not to claim that Spinoza thought biographical details were wholly irrelevant to the interpretation of his texts (as they are, say, in the interpretation of Euclid). But the biographical details that he and his friends thought that mattered are the ones that we learn primarily from his correspondence and terse autobiographical comments scattered through his texts. These letters were carefully selected/culled and generate a textual, composite portrait published alongside his other writings.
Second, except for the apparent likeness to what we think Spinoza looked like, the Graat-Spinoza portrait fails to connect Spinoza's personal or intellectual biography. The portrait does not include lenses or signs of lens-crafting; nor does the painting connect to Spinoza's then most important claim to fame: the 1663 Principia philosophiae cartesianae. The man portrayed is neither very learned nor obviously one of the philosophical Cartesian novatores (as, say, Leibniz saw him before he read Spinoza's Theological Political Treatise).
Third, the background includes a sculpture, fantasy architecture, a pyramid, and a mountain-landscape. I honestly do not see how this connects to either Cartesian or Spinozist themes of the day; if anything Spinoza's philosophy has warnings against taking the fantasies of the imagination seriously. The sculpture seems to hold a Sun-image in her hand, and maybe one can connect that with light-giving philosophy. But we're far removed from the inner light that Descartes and Spinoza preferred; this sun image is more akin to the imagery one associates with free-masons. I am not denying that there are neo-Platonic elements in Spinoza's thought (I have argued for that myself), but the solar image in this portrait is not associated with knowledge or astronomy, etc. (And again, Spinoza's lenses were also used in astronomy in the period.)
Fourth, even if we allow that despite his known skepticism about paintings being mute and passive (recall), Spinoza was vain enough to commission such a portrait or to agree to have such a portrait commissioned, he was presumably too poor to pay for it himself at the time. So, this must have been commissioned by an admirer. But the known admirers of the period are all folk that were attracted to Spinoza for either his ability to engage with their theological concerns, or to provide them with a guide to life. Again, right now I do not see any reason to connect this painting to such themes. So, until there is new evidence, I remain skeptical of the claim that Spinoza is being portrayed in the "Graat-Spinoza.' [UPDATE: For more on this issue, see here.]