Considering secession was at the time illegal, in short yes. Plain and simple. Look at Texas v. White. It was ruled unconstitutional but did however hint towards the idea that a group of states could declare independence in the event of a revolution or collective consent of the states. Of course, you could always argue the most American thing there is is in fact revolution, considering that was effectively the birthplace of the United States. Secession in the US is a somewhat controversial topic and it has been debated over the centuries but legally it is not permitted.

To be quite honest there is no way to really answer the question on the topic of treason: the entire topic is a legal quagmire and is the topic of quite a bit of scholarly debate. They Consitution never directly addressed secession and since it was in effect not an explicit right there was no real punishment for it. Jefferson Davis was charged with treason but nothing ever became of it due to no internal consensus and frankly the US having bigger fish to fry in the reconstruction of the South and he was released after 2 years. So the question has never really been decided.

On a personal note I think it depends entirely on whether or not you get away with it. "History is written by the victor" and whatnot. Nobody would dare to call Washington a traitor (in America at least) but some may be quick to jump to the conclusion of Davis being a traitor. Just something to think about.

Sources:

https://www.law.cornell.edu/supremecourt/text/74/700

^ Barney, William L. (2011). The Oxford Encyclopedia of the Civil War. Oxford University Press.

I'd like to supplement /u/Crisiss's answer, especially since I respectfully disagree with him about the application of treason.

TL; DR - Yes.

"Treason against the United States, shall consist only in levying War against them, or in adhering to their Enemies, giving them Aid and Comfort. No Person shall be convicted of Treason unless on the Testimony of two Witnesses to the same overt Act, or on Confession in open Court." -U.S. Constitution, Art. III, § 3, cl. 1

Armed rebellion against the United States is the first form of treason. It should be noted that the second form doesn't actually apply at all: the Confederacy was not an "enemy" of the United States in this sense. Thorington v. Smith, 75 U.S. 1 (1868). That category only applies to subjects of a foreign power in a state of open hostility with the United States; rebels who are purporting to be a separate nation (but aren't, because secession is unconstitutional and legally invalid) don't meet that. US v. Greathouse, 26 F.Cas. 18 (C.C.N.D.Cal. 1863); Stephan v. United States, 133 F.2d 87 (6th Cir. 1943); etc.

So we're only talking about levying war against the United States. And in this, at least, the law overlaps nicely with our intuitions: participating in the Confederacy’s armed rebellion would constitute treason. At a fundamental level, after all, if massive armed rebellion against federal authority isn’t treason, what is, you know? There are some fine lines involved: for instance, resisting the execution of a federal law by armed force, if done for a private purpose, isn’t treason (US v. Hanway, 26 F.Cas. 105 (C.C.E.D.Pa. 1851)), but a group of people acting violently together to ‘suppress an office of excise and to compel the resignation of the excise officer, so as to render an Act of Congress, in effect, null and void’ is treason (US v. Vigol, 2 U.S. 346 (C.C.D.Pa. 1795)).

But that digression aside, the core nature of this form of treason, i.e. levying war against the United States, is: the assembly of a group of people for a treasonable purpose (that is, overthrowing the entire national government, overthrowing the government in certain parts of the country, or defeating the execution and forcing the repeal of one of its laws), with an overt act of some kind done towards that end. For assemblages of this nature which don’t actually end up launching a rebellion, they also need to be in a condition to make war against the government and have the intention to do so (more on that below), but the aforementioned elements are what really defines levying war against the United States. There can be no serious question that agents of the Confederacy satisfy those elements. Ordinary citizens of the South, probably not; but soldiers and leaders, very likely.

Note that mere conspiracy is not enough for treason: actual levying of war is required. Ex parte Bollman, 8 U.S. 75 (1807). So if a group of people meets to plan the overthrow of the government, that’s not treason. But crucially, levying of war (and therefore treason) does not require that you actually carry out your armed rebellion: assembling a group of people with the intent of making war against the government and the capability of doing so is an act of levying war, even if no violence is actually committed. US v. Burr, 25. F.Cas. 55 (C.C.D.Va. 1807). An overt act of some kind towards the treasonable purpose is required, but not necessarily the commencement of actual fighting. A particularly good, yet concise, explanation of this sort of “incipient rebellion” treason comes from the revered Justice Story, at In re Charge to Grand Jury, 30 F.Cas. 1046 (C.C.R.I. 1842). The upshot of this, you might say, is that failed rebellions are still treasonous.

There is also no such thing as an accessory to treason: anyone who participates is a full traitor. That is, anyone who’s a part of the general conspiracy to overthrow the government, no matter how small their part or how distant they are from the actual assemblage of treasonously-intentioned people, is guilty of treason as well. US v. Burr, 25 F.Cas. 55 (C.C.D.Va. 1807); Tomoya Kawakita v. US, 343 U.S. 717 (1952); etc.

As for Jefferson Davis, there’s actually quite a strong legal case against him for treason. According to William J. Cooper’s book Jefferson Davis, American, at p. 602-03, the problems with prosecuting him were practical in nature: for one thing, the plan was to do it in Virginia, where he had actually overseen armed rebellion against the United States, but there was serious concern about whether any jury of Virginians would actually convict Jefferson Davis of treason. But I can’t claim any real knowledge of the circumstances around the indictment, and eventual non-prosecution, of Jefferson Davis. Or any Confederate leaders, for that matter. I’m just focusing on the legal stuff here, and while I don't myself have the knowledge to explain why there wasn't widespread prosecution for treason if the legal issues were as relatively clear-cut as I've suggested, I suspect it's explained by the desire for reconciliation and the practical difficulties of holding proper (i.e. trials in the South, by a jury of southerners) yet successful treason trials for Confederate leaders.

EDIT: To clarify the case citations, by the way, C.C. stands for Circuit Court. So C.C.D.Va. is the Circuit Court for the District of Virginia, a federal lower court (now obsolete). The ones with U.S. in the citation are decisions by the Supreme Court, except for the Vigol case, which is (for whatever reason) a lower court decision recorded in the same case reporter the Supreme Court uses. And the Stephan case is the only example of this, but 6th Cir. stands for the Sixth Circuit Court of Appeals, a modern federal appellate court with jurisdiction over the federal district courts in Ohio, Tennessee, Michigan, and Kentucky.