Two Angolans, sports journalist Amor Carlos Tomé and political activist Francisco Oliveira, were also on trial on similar charges for their alleged part in the Russian-linked influence operation, which they both denied.
Tomé received a two-year suspended sentence and Oliveira was acquitted.
Prosecutors appealed immediately after the judgement, which was read out after 23:00 local time on Tuesday.
Tomé is expected to remain in custody pending the outcome of the appeal.
The BBC has contacted the Russian embassy in Angola for comment but had not received a response at the time of publication.
Elizeu Nguiniti, the lawyer representing Ratchin and Lakshtanov, told the BBC he would not comment on the decision.
After the judgement was read out, he told the court he disagreed with the verdict, which found his clients guilty of four of 11 charges – criminal association, espionage, terrorism and illegal retention of currency – but said he accepted the court’s decision.
According to Angola’s privately owned news site Novo Jornal, the Russians told the court during previous hearings they had held private meetings with political figures from both the ruling MPLA and opposition Unita parties, but denied that the meetings were connected to any illegal activity.
It reported that the court had rejected a defence request to call several of those politicians as witnesses, ruling that their testimony was “not necessary” for establishing the facts of the case.
Prosecutors alleged that the defendants played a role in organising deadly unrest in Angola in 2025.
They alleged the Russians paid local journalists and experts more than $24,000 (£18,000) to spread pro-Russian and anti-government content aimed at provoking political change in Angola.
They also cited publications linked to the operation, notes and photographs related to the 2025 protests found on the defendants’ devices, as well as alleged contacts with senior Angolan political figures.
Many Angolan activists strongly disagree with the prosecution’s argument that the protests did not happen organically.
Novo Jornal previously reported that Lakshtanov told the court during one of the hearings that he had lived and worked in Angola for nearly four decades and that his long academic career was incompatible with participation in terrorist organisations.
Earlier Ratchin had explained to the court that he was a businessman who had come to Angola to establish a Russian cultural centre and that payments he made to various individuals were compensation for services rendered, Novo Jornal reported.
He reportedly said he was surprised by his arrest in August 2025 and the subsequent charges.
Early proceedings of the trial, which began in March, were marked by a controversy.
During one of the initial hearings, the prosecutor declined to read the indictment aloud in court. She argued that the defendants were already aware of the accusations against them and therefore formal reading was not required.
The defence disagreed. They argued that since the trial was public, those attending the proceedings, including journalists, should be able to hear and understand the allegations.
Nevertheless, the prosecutor maintained that reading the indictment was optional and refused to do so.
The indictment itself had been criticised even before the trial began.
The defence has since consistently challenged its contents, arguing it often relied on assumptions rather than concrete evidence.
The BBC had previously identified a number of misspellings and apparent factual errors in the document, but received no clarification from the prosecution about these.
Additional reporting by Ilya Barabanov and BBC Russian’s Investigations Team