The quaternion Fourier transform (QFT) satisfies some uncertainty principles similar to the Euclidean Fourier transform. In this paper, we establish Miyachi's theorem for this transform and consequently generalize and prove the analogue of Hardy's theorem and Cowling-Price uncertainty principle in the QFT domain.1 done, that is the so-called left-sided, right-sided and the two-sided QFT. The QFT was introduced at first by Ell [10] for the analysis linear time-invariant partial differential systems and then applied in color image processing. Later, Blow [3] investigated the important properties of the two-sided QFT for real signals and applied it to signal and image processing. Furthermore, several uncertainty principles have been formulated for the quaternion Fourier transform.In [15], the authors generalized a component-wise UP for the right-sided QFT. The directional UP related to the two-sided QFT was proposed in [12]. Recently,in [4], the authors established logarithmic UP associated with the QFT. Meanwhile, Mawardi [16] obtained the connection between the QFT and quantum mechanics and then established the modified UP (full UP) for the two-sided QFT.Our contribution to these developments is that we propose a new UP for the QFT, namely Miyachi's UP. So far, no such uncertainty principle for the QFT (one-sided or two-sided) had been established. In our previous works, other UPs: Heisenberg, Hardy[7], and Beurling[8], have been extended for the two-sided QFT. Also, we derived in [8] the UPs of Cowling-Price and Hardy using the extension of Beurling theorem in a quaternion framework. Here, we will obtain, in a different way, by the main result of Miyachi, the same UPs of Cowling-Price and Hardy in QFT domain. The techniques used here are also applicable for the left-sided and the right-sided QFT as well. Our paper is organized as follows. In Sect. 2, we review some basic notions and notations related to the quaternion algebra. In Sect. 3, we recall the definition and some results for the quaternion Fourier transform useful in the sequel. In Sect. 4, we prove Miyachis theorem for the quaternion Fourier transform, and provide an extension of certain UPs to the quaternion Fourier transform domain. In Sect. 5, we conclude the paper.